Semiconductor optical device

ABSTRACT

In a semiconductor optical device, a first conductive type semiconductor region is provided on a surface of GaAs. The first conductive type semiconductor region has a first region and a second region. An active layer is provided on the first region of the first conductive type semiconductor region. The active layer has a pair of side surfaces. A second conductive type semiconductor region is provided on the sides and top of the active layer, and the second region of the first conductive type semiconductor region. The bandgap energy of the first conductive type semiconductor region is greater than that of the active layer. The bandgap energy of the second conductive type semiconductor region is greater than that of the active layer. The second region of the first conductive type semiconductor region and the second conductive type semiconductor region constitute a pn junction.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor optical device.

2. Related Background of the Invention

There are a variety of structures of semiconductor optical devices, suchas a semiconductor laser. For example, publication 1 (IEEE JOURNAL OFQUANTUM ELECTRONICS, VOL. QE-17, NO. 2, FEBRUARY 1981, pp. 202-207)discloses a buried hetero-structure semiconductor laser. Thissemiconductor laser has an active layer made of GaInAsP semiconductor.This active layer is provided between a p-type InP semiconductor layerand an n-type InP semiconductor layer and is located between InP currentblock portions of the InP semiconductor layer. Carriers are injectedinto the active layer and the injected carriers are confined into theactive layer by the hetero-barriers at the interfaces between the activelayer and the current block portions.

SUMMARY OF THE INVENTION

In semiconductor optical devices, such as buried hetero-structure typesemiconductor laser as disclosed above, the current vs. optical outputpower characteristics depend mainly on bandgap energies of the activelayer and other semiconductor layers and cannot be changed in accordancewith their applications. The inventors have thought that opticalsemiconductor devices can be used for wider range of applications if itscurrent vs. optical output power characteristics can be changed.

It is an object of the present invention to provide an opticalsemiconductor device having current vs. optical output powercharacteristics which can be changed.

According to one aspect of the present invention, a semiconductoroptical device comprises a first conductive type semiconductor region,an active layer, a second conductive type semiconductor region and apotential adjusting semiconductor layer. The first conductive typesemiconductor region has a first semiconductor portion and a secondsemiconductor portion. The first and second semiconductor portions areprovided along a predetermined surface. The first semiconductor portionhas a first region and a second region. The second semiconductor portionhas a pair of sides. The second semiconductor portion is provided on thefirst region of the first semiconductor portion. The active layer isprovided on the second semiconductor portion of the first conductivetype semiconductor region. The active layer has a pair of sides. Thesecond conductive type semiconductor region is provided on the sides andtop of the active layer, the sides of the second semiconductor portion,and the second region of the first semiconductor portion of the firstconductive type semiconductor region. A bandgap energy of the firstconductive type semiconductor region is greater than that of the activelayer. A bandgap energy of the second conductive type semiconductorregion is greater than that of the active layer. The second region ofthe first semiconductor portion of the first conductive typesemiconductor region and the second conductive type semiconductor regionconstitute a pn junction. The potential adjusting semiconductor layer isprovided between the second semiconductor portion of the firstconductive type semiconductor region and the active layer. A bandgapenergy of the potential adjusting semiconductor layer is different fromthat of the first conductive type semiconductor region. The bandgapenergy of the potential adjusting semiconductor layer is different fromthat of the second conductive type semiconductor region.

According to another aspect of the present invention, a semiconductoroptical device comprises a first conductive type semiconductor region,an active layer, a second conductive type semiconductor region, and apotential adjusting semiconductor layer. The first conductive typesemiconductor region has a first semiconductor portion and a secondsemiconductor portion. The first and second semiconductor portions areprovided along a predetermined surface. The first semiconductor portionhas a first region and a second region. The second semiconductor portionhas a pair of sides. The second semiconductor portion is provided on thefirst region of the first semiconductor portion. The active layer isprovided on the second semiconductor portion of the first conductivetype semiconductor region. The active layer has a pair of sides. Thesecond conductive type semiconductor region is provided on the sides andtop of the active layer, the sides of the second semiconductor portion,and the second region of the first semiconductor portion of the firstconductive type semiconductor region. A bandgap energy of the firstconductive type semiconductor region is greater than that of the activelayer. A bandgap energy of the second conductive type semiconductorregion is greater than that of the active layer. The second region ofthe first semiconductor portion of the first conductive typesemiconductor region and the second conductive type semiconductor regionconstitute a pn junction. The potential adjusting semiconductor layer isprovided between the second conductive type semiconductor region and theactive layer. A bandgap energy of the potential adjusting semiconductorlayer is different from that of the first conductive type semiconductorregion. The bandgap energy of the potential adjusting semiconductorlayer is different from that of the second conductive type semiconductorregion.

In the semiconductor optical device according to the present invention,the potential adjusting semiconductor layer is provided between thesecond semiconductor portion of the first conductive type semiconductorregion and the active layer.

According to still another aspect of the present invention, asemiconductor optical device comprises a first conductive typesemiconductor region, an active layer, a second conductive typesemiconductor region, and a potential adjusting semiconductor layer. Thefirst conductive type semiconductor region has a first region and asecond region. The first and second regions are provided along apredetermined surface. The active layer is provided on the first regionof the first conductive type semiconductor region. The active layer hasa pair of sides. The second conductive type semiconductor region isprovided on the sides and top of the active layer and the second regionof the first conductive type semiconductor region. The potentialadjusting semiconductor layer is provided between the first region ofthe first conductive type semiconductor region and the active layer. Abandgap energy of the potential adjusting semiconductor layer isdifferent from that of the first conductive type semiconductor region.The bandgap energy of the potential adjusting semiconductor layer isdifferent from that of the second conductive type semiconductor region.The bandgap energy of the first conductive type semiconductor region isgreater than that of the active layer. The bandgap energy of the secondconductive type semiconductor region is greater than that of the activelayer. The second region of the first conductive type semiconductorregion and the second conductive type semiconductor region constitute apn junction.

According to another aspect of the present invention, a semiconductoroptical device comprises a first conductive type semiconductor region,an active layer, a second conductive type semiconductor region, and apotential adjusting semiconductor layer. The first conductive typesemiconductor region has a first region and a second region. The firstand second regions are provided along a predetermined surface. Theactive layer is provided on the first region of the first conductivetype semiconductor region. The active layer has a pair of sides. Thesecond conductive type semiconductor region is provided on the sides andtop of the active layer and the second region of the first conductivetype semiconductor region. The potential adjusting semiconductor layeris provided between the second conductive type semiconductor region andthe active layer. A bandgap energy of the potential adjustingsemiconductor layer is different from that of the first conductive typesemiconductor region. The bandgap energy of the potential adjustingsemiconductor layer is different from that of the second conductive typesemiconductor region. The bandgap energy of the first conductive typesemiconductor region is greater than that of the active layer. Thebandgap energy of the second conductive type semiconductor region isgreater than that of the active layer. The second region of the firstsemiconductor portion of the first conductive type semiconductor regionand the second conductive type semiconductor region constitute a pnjunction.

In the semiconductor optical device according to the present invention,the potential adjusting semiconductor layer is provided between thefirst region of the first conductive type semiconductor region and theactive layer.

According to another aspect of the present invention, a semiconductoroptical device comprises a first conductive type semiconductor region,an active layer, a second conductive type semiconductor region, and apotential adjusting semiconductor layer. The first conductive typesemiconductor region has a first semiconductor portion and a secondsemiconductor portion. The first and second semiconductor portions areprovided along a predetermined surface. The first semiconductor portionhas a first region and a second region. The second semiconductor portionhas a pair of sides. The second semiconductor portion is located on thefirst region of the first semiconductor portion. The active layer isprovided on the second semiconductor portion of the first conductivetype semiconductor region. The active layer has a pair of sides. Thesecond conductive type semiconductor region is provided on the sides andtop of the active layer, the pair of sides of the second semiconductorportion, and the second region of the first semiconductor portion of thefirst conductive type semiconductor region. A bandgap energy of thefirst conductive type semiconductor region is greater than that of theactive layer. A bandgap energy of the second conductive typesemiconductor region is greater than that of the active layer. Thepotential adjusting semiconductor layer is provided between the secondregion of the first semiconductor portion of the first conductive typesemiconductor region and the second conductive type semiconductorregion. A bandgap energy of the potential adjusting semiconductor layeris different from that of the first conductive type semiconductorregion. The bandgap energy of the potential adjusting semiconductorlayer is different from that of the second conductive type semiconductorregion. The second region of the first semiconductor portion of thefirst conductive type semiconductor region, the second conductive typesemiconductor region and the potential adjusting semiconductor layer arearranged to form a pn junction therein.

According to another aspect of the present invention, a semiconductoroptical device comprises a first conductive type semiconductor region,an active layer, a second conductive type semiconductor region, and apotential adjusting semiconductor layer. The first conductive typesemiconductor region has a first region and a second region. The firstand second regions are provided along a predetermined surface. Theactive layer is provided on the first region of the first conductivetype semiconductor region. The active layer has a pair of sides. Thesecond conductive type semiconductor region is provided on the sides andtop of the active layer, and the second region of the first conductivetype semiconductor region. The potential adjusting semiconductor layeris provided between the second region of the first conductive typesemiconductor region and the second conductive type semiconductorregion. The bandgap energy of the potential adjusting semiconductorlayer is different from that of the first conductive type semiconductorregion. The bandgap energy of the potential adjusting semiconductorlayer is different from that of the second conductive type semiconductorregion. The second region of the first conductive type semiconductorregion, the second conductive type semiconductor region and thepotential adjusting semiconductor layer are arranged to form a pnjunction therein.

In the semiconductor optical device according to the present invention,the potential adjusting semiconductor layer includes a first region of afirst conductive type and a second region of a second conductive type.The first region and second region of the potential adjustingsemiconductor layer constitute the pn junction. The first region of thepotential adjusting semiconductor layer and the second region of thefirst conductive type semiconductor region constitute a junction. Thesecond region of the potential adjusting semiconductor layer and thesecond conductive type semiconductor region constitute a junction.

According to another aspect of the present invention, a semiconductoroptical device comprises a first conductive type semiconductor region,an active layer, a second conductive type semiconductor region and apotential adjusting semiconductor layer. The first conductive typesemiconductor region has a first semiconductor portion and a secondsemiconductor portion. The first semiconductor portion has a firstregion and a second region. The first and second regions are providedalong a predetermined surface. The second semiconductor portion has apair of sides. The second semiconductor portion is located on the firstregion of the first semiconductor portion. The active layer is providedon the second semiconductor portion of the first conductive typesemiconductor region. The active layer has a pair of sides. The secondconductive type semiconductor region is provided on the sides and top ofthe active layer, the sides of the second semiconductor portion and thesecond region of the first semiconductor portion of the first conductivetype semiconductor region. A bandgap energy of the first conductive typesemiconductor region is greater than a bandgap energy of the activelayer. A bandgap energy of the second conductive type semiconductorregion is greater than the bandgap energy of the active layer. Thepotential adjusting semiconductor layer is provided between the secondregion of the first semiconductor portion of the first conductive typesemiconductor region and the second conductive type semiconductor regionand between the second semiconductor portion of the first conductivetype semiconductor region and the active layer. A bandgap energy of thepotential adjusting semiconductor layer is different from that of thefirst conductive type semiconductor region. The bandgap energy of thepotential adjusting semiconductor layer is different from that of thesecond conductive type semiconductor region. The second region of thefirst semiconductor portion of the first conductive type semiconductorregion, the second conductive type semiconductor region and thepotential adjusting semiconductor layer are arranged to form a pnjunction therein.

According to another aspect of the present invention, a semiconductoroptical device comprises a first conductive type semiconductor region,an active layer, a second conductive type semiconductor region and apotential adjusting semiconductor layer. The first conductive typesemiconductor region has a first semiconductor portion and a secondsemiconductor portion. The first semiconductor portion has a firstregion and a second region. The first and second regions are providedalong a predetermined surface. The second semiconductor portion has apair of sides. The second semiconductor portion is located on the firstregion of the first semiconductor portion. The active layer is providedon the second semiconductor portion of the first conductive typesemiconductor region. The active layer has a pair of sides. The secondconductive type semiconductor region is provided on the sides and top ofthe active layer, the sides of the second semiconductor portion and thesecond region of the first semiconductor portion of the first conductivetype semiconductor region. Bandgap energies of the second conductivetype semiconductor region and the first conductive type semiconductorregion are greater than a bandgap energy of the active layer. Thepotential adjusting semiconductor layer is provided between the secondregion of the first semiconductor portion of the first conductive typesemiconductor region and the second conductive type semiconductor regionand between the second conductive type semiconductor region and theactive layer. A bandgap energy of the potential adjusting semiconductorlayer is different from that of the first conductive type semiconductorregion. The bandgap energy of the potential adjusting semiconductorlayer is different from that of the second conductive type semiconductorregion. The second region of the first semiconductor portion of thefirst conductive type semiconductor region, the second conductive typesemiconductor region and the potential adjusting semiconductor layer arearranged to form a pn junction therein.

In the semiconductor optical device according to the present invention,the potential adjusting semiconductor layer is provided between thesecond semiconductor portion of the first conductive type semiconductorregion and the active layer.

According to another aspect of the present invention, a semiconductoroptical device comprises a first conductive type semiconductor region,an active layer, a second conductive type semiconductor region, and apotential adjusting semiconductor layer. The first conductive typesemiconductor region has a first region and a second region. The firstand second regions are provided along a predetermined surface. Theactive layer is provided on the first region of the first conductivetype semiconductor region. The active layer has a pair of sides. Thesecond conductive type semiconductor region is provided on the sides andtop of the active layer and the second region of the first conductivetype semiconductor region. A bandgap energy of the first conductive typesemiconductor region is greater than that of the active layer and abandgap energy of the second conductive type semiconductor region isgreater than that of the active layer. The potential adjustingsemiconductor layer is provided between the second region of the firstconductive type semiconductor region and the second conductive typesemiconductor region and between the first region of the firstconductive type semiconductor region and the active layer. A bandgapenergy of the potential adjusting semiconductor layer is different fromthat of the first conductive type semiconductor region, and the bandgapenergy of the potential adjusting semiconductor layer is different fromthat of the second conductive type semiconductor region. The secondregion of the first conductive type semiconductor region, the secondconductive type semiconductor region and the potential adjustingsemiconductor layer are arranged to form a pn junction therein.

According to another aspect of the present invention, a semiconductoroptical device comprises a first conductive type semiconductor region,an active layer, a second conductive type semiconductor region, and apotential adjusting semiconductor layer. The first conductive typesemiconductor region has a first region and a second region. The firstand second regions are provided along a predetermined surface. Theactive layer is provided on the first region of the first conductivetype semiconductor region. The active layer has a pair of sides. Thesecond conductive type semiconductor region is provided on the sides andtop of the active layer and the second region of the first conductivetype semiconductor region. A bandgap energy of the first conductive typesemiconductor region is greater than that of the active layer, and abandgap energy of the second conductive type semiconductor region isgreater than that of the active layer. The potential adjustingsemiconductor layer is provided between the second region of the firstconductive type semiconductor region and the second conductive typesemiconductor region and between the second conductive typesemiconductor region and the active layer. A bandgap energy of thepotential adjusting semiconductor layer is different from that of thefirst conductive type semiconductor region. The bandgap energy of thepotential adjusting semiconductor layer is different from that of thesecond conductive type semiconductor region. The second region of thefirst conductive type semiconductor region, the second conductive typesemiconductor region and the potential adjusting semiconductor layer arearranged to form a pn junction therein.

In the semiconductor optical device according the present invention, thepotential adjusting semiconductor layer is provided between the firstregion of the first conductive type semiconductor region and the activelayer.

In the semiconductor optical device according to the present invention,the potential adjusting semiconductor layer is made of materialresistant to an etchant for etching the active layer, and the materialpermits the potential adjusting semiconductor layer to work as anetching stop layer for etching the active layer.

In the semiconductor optical device according to the present invention,the bandgap energy of the potential adjusting semiconductor layer issmaller than bandgap energies of the first and second conductive typesemiconductor regions, and the bandgap energy of the potential adjustingsemiconductor layer is larger than that of the active layer.

In the semiconductor optical device according to the present invention,the bandgap energy of the potential adjusting semiconductor layer islarger than bandgap energies of the first and second conductive typesemiconductor regions.

In the semiconductor optical device according to the present invention,the first conductive type semiconductor region is made of at least oneof AlGaAs, AlGaInP, GaInP and GaInAsP, the second conductive typesemiconductor region is made of at least one of AlGaAs, AlGaInP, GaInPand GaInAsP, the active layer is made of GaInNAs, and the potentialadjusting layer is made of at least one of AlGaInP, GaInP, AlGaAs, GaAs,GaInAsP and GaInAs.

In the semiconductor optical device according to the present invention,the semiconductor optical device further comprises a contact layerprovided only on a part of on the active layer and the second conductivetype semiconductor region and above the active layer. A width of thecontact layer is not more than that of the active layer.

In the semiconductor optical device according to the present invention,the first conductive type semiconductor region includes a concentrationchanging region and another region, the concentration changing region ofthe first conductive type semiconductor region is provided between theother region of the first conductive type semiconductor region and thesecond conductive type semiconductor region, the concentration changingregion of the first conductive type semiconductor region contacts withthe second conductive type semiconductor region, and a dopantconcentration of the concentration changing region of the firstconductive type semiconductor region is different from the other regionof the first conductive type semiconductor region.

In the semiconductor optical device according to the present invention,the second conductive type semiconductor region includes a concentrationchanging region and another region, the concentration changing region ofthe second conductive type semiconductor region is provided between theanother region of the second conductive type semiconductor region andthe first conductive type semiconductor region, the concentrationchanging region of the second conductive type semiconductor regioncontacts with the first conductive type semiconductor region, and adopant concentration of the concentration changing region of the secondconductive type semiconductor region is different from the other regionof the second conductive type semiconductor region.

In the semiconductor optical device according to the present invention,the semiconductor optical device further comprises a first opticalconfinement layer provided between the first conductive typesemiconductor region and the active layer; and a second opticalconfinement layer provided between the second conductive typesemiconductor region and the active layer.

In the semiconductor optical device according to the above aspects, thesemiconductor optical device includes a semiconductor laser. In thesemiconductor optical device according to the above aspects, thesemiconductor optical device includes a light emitting diode. In thesemiconductor optical device according to the above aspects, thesemiconductor optical device includes a semiconductor optical amplifier.In the semiconductor optical device according to the above aspects, thesemiconductor optical device includes an electro-absorption typemodulator. In the semiconductor optical device according to the aboveaspects, the semiconductor optical device includes a semiconductoroptical waveguide.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-described object and other objects, features, and advantagesof the present invention will become apparent more easily in thedetailed description of the preferred embodiments of the presentinvention which will be described below with reference to theaccompanying drawings.

FIG. 1 is a perspective view showing a semiconductor optical deviceaccording to the first embodiment.

FIG. 2 shows a cross sectional view taken along line I-I in FIG. 1 and abandgap diagram taken along line II-II in FIG. 2.

FIG. 3 is an equivalent circuit diagram showing the electrical propertyof the semiconductor optical device according to the first embodiment.

FIG. 4 schematically shows current vs. voltage and current vs. opticaloutput power characteristics for the semiconductor optical deviceaccording to the first embodiment.

FIG. 5 is a view for explaining the fabrication of the semiconductoroptical device.

FIG. 6 schematically shows graphs of the following: the relationshipbetween the applied voltage and applied current; the relationshipbetween the applied current and optical output power of the opticalsemiconductor optical device which generates optical output pulses inresponse to applied pulsed current.

FIG. 7 is a view showing a first modified semiconductor optical deviceaccording to the first embodiment.

FIG. 8 schematically shows current vs. voltage characteristics andcurrent vs. optical output power characteristics for the semiconductoroptical device according to the first modified semiconductor opticaldevice.

FIG. 9 is a view showing a second modified semiconductor optical deviceaccording to the first embodiment.

FIG. 10 is a view showing a third modified semiconductor optical deviceaccording to the first embodiment.

FIG. 11 is a view showing the third modified semiconductor opticaldevice according to the first embodiment.

FIG. 12 is a view showing the third modified semiconductor opticaldevice according to the first embodiment.

FIG. 13 is a perspective view showing a semiconductor optical deviceaccording to the second embodiment.

FIG. 14 is a cross sectional view, taken along III-III line shown inFIG. 13, showing a semiconductor optical device according to the secondembodiment.

FIG. 15 is a view showing a fourth modified semiconductor optical deviceaccording to the second embodiment.

FIG. 16 is a view showing a fifth modified semiconductor optical deviceaccording to the second embodiment.

FIG. 17 is a view showing a sixth modified semiconductor optical deviceaccording to the second embodiment.

FIG. 18 is a view showing a seventh modified semiconductor opticaldevice according to the second embodiment.

FIG. 19 is a view showing an eighth modified semiconductor opticaldevice according to the second embodiment.

FIG. 20 is a view showing the eighth modified semiconductor opticaldevice according to the second embodiment.

FIG. 21 is a view showing the eighth modified semiconductor opticaldevice according to the second embodiment.

FIG. 22 is a view showing the eighth modified semiconductor opticaldevice according to the second embodiment.

FIG. 23 is a view showing the eighth modified semiconductor opticaldevice according to the second embodiment.

FIG. 24 is a view showing a ninth modified semiconductor optical deviceaccording to the second embodiment.

FIG. 25 schematically shows the following: the relationship betweencurrent vs. voltage characteristics and the relationship between currentvs. optical output characteristics for the semiconductor optical deviceaccording to the ninth modified semiconductor optical device.

FIG. 26 is a view showing a list of materials for semiconductor layersin the semiconductor optical device according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The teachings of the present invention will readily be understood inview of the following detailed description with reference to theaccompanying drawings illustrated by way of example. When possible,parts identical to each other will be referred to with reference symbolsidentical to each other.

First Embodiment

FIG. 1 is a perspective view showing a semiconductor optical deviceaccording to the first embodiment. An XYZ coordinate system S isdepicted in FIG. 1. FIG. 2 is a view for the semiconductor opticaldevice according to the first embodiment. Area (a) in FIG. 2 shows across sectional view taken along line I-I in FIG. 1. Area (b) in FIG. 2shows one example of band diagrams, taken along line II-II in area (a),for the semiconductor optical device according to the first embodiment.Area (c) in FIG. 2 shows another example of band diagrams, taken alongline II-II in area (a), for the semiconductor optical device accordingto the first embodiment.

FIGS. 1 and 2 show a semiconductor optical device 1, such as asemiconductor laser. The semiconductor optical device 1 comprises apotential adjusting layer 2, a first conductive type semiconductorregion 3, an active layer 5, and a second conductive type semiconductorregion 7. The first conductive type semiconductor region 3 is providedon the surface of semiconductor, such as GaAs semiconductor, InPsemiconductor, GaN semiconductor and SiC semiconductor. The firstconductive type semiconductor region 3 has first and secondsemiconductor portions 3 a and 3 b. As shown in FIG. 2, the firstsemiconductor portion 3 a includes a first region 3 c and a secondregion 3 d located on both sides of the first region 3 c. The firstregion 3 c extends in the direction of the z-axis. The secondsemiconductor portion 3 b is located on the first region 3 c of thefirst semiconductor portion 3 a. The second semiconductor portion 3 bhas a pair of sides 3 e. The active layer 5 is provided on the secondsemiconductor portion 3 b of the first conductive type semiconductorregion 3. The active layer 5 has a pair of sides 5 a. The secondconductive type semiconductor region 7 has a top surface 7 a and abottom surface 7 b. Bottom surface 7 b is provided on the second region3 d of the first semiconductor portion 3 a of the first conductive typesemiconductor region 3. Bottom surface 7 b contacts sides 3 e of thesecond semiconductor portion 3 b, and the sides 5 a of the active layer5. The potential adjusting layer 2 is provided between the active layer5 and the second conductive type semiconductor region 7. The potentialadjusting layer 2 has a pair of sides 2 e. The second conductive typesemiconductor region 7 and the second region 3 d of the firstsemiconductor portion 3 a of the first conductive type semiconductorregion 3 form a pn junction around the active layer 5. The active layeris made of III-V compound semiconductor.

The first conductive type semiconductor region 3 is made of III-Vcompound semiconductor, the bandgap energy of which is greater than thatof the active layer 5. In other words, the photoluminescence wavelengthof III-V compound semiconductor of the first conductive typesemiconductor region 3 is shorter than that of the active layer 5. Thesecond conductive type semiconductor region 7 is made of III-V compoundsemiconductor, the bandgap energy of which is greater than that of theactive layer 5. In other words, the photoluminescence wavelength ofIII-V compound semiconductor of the second conductive type semiconductorregion 7 is shorter than that of the active layer 5. Thephotoluminescence wavelength of semiconductor material is equal to awavelength corresponding to the bandgap energy thereof.

The potential adjusting layer 2 is made of III-V compound semiconductor,and areas (b) and (c) of FIG. 2 show the bandgap energy relationshipsbetween this III-V compound semiconductor and materials of the adjacentsemiconductor regions. In one relationship shown in area (b) of FIG. 2,the bandgap energy of III-V compound semiconductor of the potentialadjusting layer 2 is greater than that of the active layer 5 and issmaller than bandgap energies of the first and second conductive typesemiconductor regions 3 and 7. In other words, the photoluminescencewavelength of III-V compound semiconductor of the potential adjustinglayer 2 is shorter than that of the active layer 5 and is longer thanbandgap energies of the first and second conductive type semiconductorregions 3 and 7. In the other relationship shown in area (c) of FIG. 2,the bandgap energy of III-V compound semiconductor of the potentialadjusting layer 2 is greater than that of the active layer 5 and isgreater than bandgap energies of the first and second conductive typesemiconductor regions 3 and 7. In other words, the photoluminescencewavelength of III-V compound semiconductor of the potential adjustinglayer 2 is shorter than that of the active layer 5 and is shorter thanbandgap energies of the first and second conductive type semiconductorregions 3 and 7.

The conductivity type of the potential adjusting layer 2 is one of thefollowing: the conductivity type of the potential adjusting layer 2 canbe the same as that of the second conductive type semiconductor region7; the conductivity type of the potential adjusting layer 2 can beundoped as with the active layer 5; the conductivity type of thepotential adjusting layer 2 can be the same as that of the active layer5 if the active layer 5 has conductivity type.

As seen from the bandgap diagram shown in areas (b) and (c) of FIG. 2,the first conductive type semiconductor region 3 and second conductivetype semiconductor region 7 confine carriers to the active layer 5.Consequently, the first conductive type semiconductor region 3 works asa cladding layer of the first conductive type and the second conductivetype semiconductor region 7 works as a cladding layer of the secondconductive type. In the active layer 5, the confined carriers injectedfrom the first conductive type semiconductor region 3 and secondconductive type semiconductor region 7 are recombined to generate light.

The refractive index of the first conductive type semiconductor region 3is smaller than that of active layer 5. The refractive index of thesecond conductive type semiconductor region 7 is also smaller than thatof active layer 5. Accordingly, the first conductive type semiconductorregion 3 and the second conductive type semiconductor region 7 confinelight from the active layer 5 into the active layer 5 in both x and ydirections. Consequently, the first conductive type semiconductor region3 and the second conductive type semiconductor region 7 act as opticalcladding layers.

The active layer 5 may have the structure of the following: the bulkstructure of a single layer, the single quantum well structure of asingle quantum well layer or the multiple quantum well structure of aplurality of well layers and barrier layers which are alternatelyarranged.

The semiconductor optical device 1 further comprises a semiconductorsubstrate 11. For example, the semiconductor substrate 11 can be made ofthe following: GaAs substrates; InP substrates; GaN substrates; SiCsubstrates. One of these substrates provides the primary surface 11 a ofGaAs, InP, GaN, or SiC on which the first conductive type semiconductorregion 3 is grown. On the primary surface 11 a of the substrate 11, thefirst conductive type semiconductor region 3 is grown. In the firstconductive type semiconductor region 3, the first semiconductor portion3 a is provided on the primary surface 11 a of the semiconductorsubstrate 11, and the second semiconductor portion 3 b is provided onthe first semiconductor portion 3 a. The second semiconductor portion 3b of the first conductive type semiconductor region 3 has a ridge shape.The active layer 5 is located between the potential adjusting layer 2and the second semiconductor portion 3 b of the first conductive typesemiconductor region 3. The potential adjusting layer 2 is providedbetween the active layer 5 and the second conductive type semiconductorregion 7. The second semiconductor portion 3 b of the first conductivetype semiconductor region 3, active layer 5 and potential adjustinglayer 2 constitute a semiconductor ridge portion 13. The semiconductorridge portion 13 extends in the z-direction. In the semiconductor ridgeportion 13, respective carriers from the second conductive typesemiconductor region 7 and the second semiconductor portion 3 b of thefirst conductive type semiconductor region 3 are injected into theactive layer 5.

The semiconductor optical device 1 further comprises a contact layer 17of the second conductive type, and electrodes 21 and 23. The contactlayer 17 is provided on the second conductive type semiconductor region7. The electrode 21 is provided on the contact layer 17 and extends in adirection in which the semiconductor ridge portion 13 extends. Theelectrode 23 is provided on the backside 11 b of the semiconductorsubstrate 11. The bandgap energy of the contact layer 17 is smaller thanthat of the second conductive type semiconductor layer 7. Accordingly,the contact layer 17 and the electrode 21 can form an excellent ohmiccontact therebetween.

One example of the composition of the semiconductor optical device 1 isas follows:

First Conductive Type Semiconductor Region 3:

-   AlGaAs, AlGaInP, GaInP, GaInAsP of n-type    Active Layer 5:-   Undoped (un-) GaInNAs    Potential Adjusting Layer 2:-   AlGaInP, GaInP, AlGaAs, GaAs, GaInAsP, GaInAs    Second Conductive Type Semiconductor Region 7:-   AlGaAs, AlGaInP, GaInP, GaInAsP of p-type    Semiconductor Substrate 11:-   n-type heavily-doped GaAs substrate    Contact Layer 17:-   p-type GaAs.    A number of semiconductor materials are listed above for    semiconductor layers, and combinations of the listed semiconductor    materials can be used for the semiconductor portions 2, 3 and 7.

With reference to FIGS. 3 and 4, the operation of the semiconductoroptical device 1 will be described. FIG. 3 is an equivalent circuitdiagram showing the electrical property of the semiconductor opticaldevice 1 according to the first embodiment. Area (a) in FIG. 4 shows agraph representing a relationship between the driving voltage anddriving current applied to the semiconductor optical device 1. Area (b)in FIG. 4 shows a graph representing a relationship between the opticaloutput (optical power) and the driving current applied to thesemiconductor optical device 1. Since the bandgap energies of the firstand second conductive type semiconductor regions 3 and 7 are greaterthan the bandgap energy of the active layer 5, the built-in potential ofthe pn junction (B portion in FIG. 3) constituted by the firstconductive type semiconductor region 3 and the second conductive typesemiconductor region 7 is greater than that of the pin junction (Aportion in FIG. 3) constituted by the first conductive typesemiconductor region 3, the active layer 5, the potential adjustinglayer 2 and the second conductive type semiconductor region 7.Consequently, the pn junction in the B portion has a higher turn-onvoltage and the pin junction in the A portion has a lower turn-onvoltage. Therefore, when the driving voltage has a value between theturn-on voltages of the A portion and the B portion, only the A potionturns on and forward current flows exclusively through the A portion.

As shown in FIG. 3, the equivalent circuit of the semiconductor opticaldevice 1 includes diodes D1 and D2 connected in parallel, which areformed in the A and B portions, respectively. The turn-on voltage V_(A)(shown in area (a) of FIG. 4) of the diode D1 is determined by thebuilt-in potential in the A portion, and mainly depends on bandgapenergies of the potential adjusting layer 2 and the active layer 5. Theturn-on voltage V_(B) of the diode D2 is determined by the built-inpotential in the B portion, and mainly depends on bandgap energies ofthe first and second conductive type semiconductor region 3 and 7. Sincethe built-in potential of the B portion is higher than that of the Aportion, the turn-on voltage V_(B) of the diode D2 is greater than theturn-on voltage V_(A) of the diode D1. The greater the differencebetween the built-in potentials of the B portion and the A portion is,the greater the difference between the turn-on voltages V_(A) and V_(B)is. Symbol R1 in FIG. 3 represents an equivalent resistor in the firstconductive type semiconductor region 3, and symbol R2 in FIG. 3represents an equivalent resistor in the second conductive typesemiconductor region 7.

In order to facilitate the understanding of the operation of thesemiconductor optical device 1, the operation of a semiconductor opticaldevice which does not have the above potential adjusting layer 2 will beexplained first. As shown in curve G10 in area (a) of FIG. 4, when theforward voltage applied thereto is increased, the diode D1 is turned onat the voltage of V_(A0). After the turn-on, the resistance in the Aportion is rapidly lowered and the forward current I_(A0) flows throughthe A portion. As a result, opposite charged carriers are injected intothe active layer 5 and are recombined with each other to generate light.Semiconductor lasers having normal values of cavity loss and internalloss start to oscillate at current slightly greater than current I_(A0)and this current I_(A0) is equivalent to the semiconductor laserthreshold current. When the injected current is increased over thethreshold current, the optical output power is rapidly increased. Thediode D2 in the B portion does not turn on yet and the resistance in theB portion is still high. Therefore, the B portion functions as a currentblocking region and thus this current is confined into the A portion(the active layer 5). Besides, since the refractive index of the activelayer 5 is greater than the refractive indices of the first and secondconductive type semiconductor regions 3 and 7, light generated by theactive layer 5 is confined into the active layer 5 and its neighborhood.In this operation in which the diode D1 turns on and the diode D2 doesnot turn on as described above, the confinement of the current and lightis achieved and provides the following: the effective stimulatedemission is caused in the active layer to generates light; the laser canoscillate with low threshold current; the optical power increaseslinearly with the amount of the injected current in this region, whichis referred to as a linear operation region.

When the applied voltage reaches the turn-on voltage V_(B), the diode D2turns on. The resistance of the B portion becomes low and the drivingcurrent flows into the B portion as well as into the A portion. The Bportion of low resistance increases leakage current that does not flowthrough the active layer 5. Therefore, when the driving current exceedsthe current I_(B0) corresponding to the turn-on voltage V_(B0), theleakage current that does not contribute to the stimulated emissionbecomes large and thus the slope efficiency becomes low. As a result,the operation region in which the supplied current is greater than thecurrent I_(B0) becomes an output saturation region in which the outputpower does not increase linearly with current and the relationshipbetween the output power and the supplied current is nonlinear.

The semiconductor optical device 1 according to the present inventionincludes the potential adjusting layer 2 provided between the activelayer 5 and the second conductive type semiconductor region 7.Hetero-junctions are formed by the contact of two semiconductors eachhaving a bandgap energy different from the other, one example pf thehetero-junction is the junction constituted by the active layer 5 andthe second conductive type semiconductor region 7. For example, the banddiagram in the hetero-junction constituted by the active layer 5 and thesecond conductive type semiconductor region 7 is formed such that thequasi-Fermi level of the active layer 5 is coincident with thequasi-Fermi level of the second conductive type semiconductor region 7.Due to the band gap discontinuity at hetero-junctions, band bending,such as notch and spike, occurs at hetero-junctions, thereby forminghetero-barriers depending on the amount of the band gap discontinuity.The hetero-barriers in conduction bands work as electrical resistanceagainst electrons in the conduction band and the hetero-barriers invalence bands work as electrical resistance against holes. If thepotential adjusting layer 2 has a bandgap between that of the activelayer 5 and that of the second conductive type semiconductor region 7,the bandgap energy differences between the potential adjusting layer 2and the active layer 5 and between the potential adjusting layer 2 andthe second conductive type semiconductor region 7 are smaller than thebandgap energy differences between the active layer 5 and the secondconductive type semiconductor region 7, so that the heights of thehetero-barriers therebetween, such as notch and/or spike, are reduced.Accordingly, since the resistance between the second conductive typesemiconductor region 7 and the active layer 5 is decreased, the turn onvoltage V_(A0) is lowered to the voltage V_(A1) as shown in curve G11 inarea (a) of FIG. 4 and the slope of the curve G11 after turning on ofthe A portion, i.e., series resistance, is reduced. On the other hand,since the B portion does not include any potential adjusting layer, theturn on voltage V_(B0) of the B portion remains unchanged. Therefore,the current I_(B1) corresponding to the turn on voltage V_(B0) becomeslarger than the current I_(B0) corresponding to the turn-on voltageV_(B0) (curve G10) at a semiconductor optical device without a potentialadjusting layer. As a result, the linear operation region in the currentvs. optical output characteristics becomes wider and the semiconductoroptical device 1 can maintain a linear operation up to a high poweroutput region as shown in curve G21 in area (b) of FIG. 4. Furthermore,since the resistance in the A portion is reduced, heat generation in thesemiconductor optical device 1 is decreased, leading to higher opticalpower and higher reliability of the semiconductor optical device 1.

If the potential adjusting layer 2 has a bandgap energy greater than thebandgap energy of the second conductive type semiconductor region 7, thebandgap energy difference at the hetero-junction between the activelayer 5 and the second conductive type semiconductor region 7 areincreased, and therefore the height of the hetero-barrier therebetween,such as notch and/or spike, becomes larger than that of semiconductoroptical device 1 without a potential adjusting layer. Accordingly, sincethe resistance between the second conductive type semiconductor region 7and the active layer 5 is increased, the turn on voltage V_(A0) shiftsto the larger voltage V_(A2) as shown in curve G12 in area (a) of FIG. 4and the slope of the curve G12 after the turning-on of the A portion,i.e., series resistance, becomes larger. On the other hand, since the Bportion does not include any potential adjusting layer, the turn onvoltage V_(B0) of the B portion remains unchanged. Therefore, as shownin curve G12, the B portion turns on at the current I_(B2) which issmaller than the current I_(B0) corresponding to a turn on voltageV_(B0) of a semiconductor optical device without a potential adjustinglayer. As a result, as shown in curve G22 in area (b) of FIG. 4, thesaturation of the optical output power occurs in a lower optical outputregion of the current vs. optical output characteristics.

With reference to FIG. 5, the fabrication of the semiconductor opticaldevice will be explained. As shown in area (a) in FIG. 5, a firstconductive type semiconductor layer 12, an active layer 14, a potentialadjusting layer 10, a second conductive type semiconductor layer 16 anda protect layer 19 are grown on the semiconductor substrate 11 made ofGaAs (the first crystal growth step). The above layers can be grownusing Organo-Metallic Vapor Phase Epitaxy (OMVPE) method or MolecularBeam Epitaxy (MBE) method, for example. Then, an etching mask 20 isformed on the protect layer 19 for forming a stripe-shaped semiconductorridge portion 13. For example, the material of the etching mask 20 canbe made of insulator, such as SiN or SiO₂.

As shown in area (b) of FIG. 5, the protect layer 19, the secondconductive type semiconductor layer 16, the potential adjusting layer10, the active layer 14, the first conductive type semiconductor layer12 are partially etched by wet etching or dry etching using the etchingmask 20 to form the second conductive type semiconductor layer 7 a, thepotential adjusting layer 2, the active layer 5 and the secondsemiconductor portion 3 b of the first conductive type semiconductorregion 3. After the etching, the sides 7 e of the second conductive typesemiconductor layer 7 a, the sides 2 e of the potential adjusting layer2, the sides 5 a of the active layer 5 and the sides 3 e of the secondsemiconductor portion 3 b of the first conductive type semiconductorregion 3 are formed. The semiconductor ridge portion 13 includes thepotential adjusting layer 2, the active layer 5 and the secondsemiconductor portion 3 b of the first conductive type semiconductorregion 3. Area (b) in FIG. 5 shows the semiconductor ridge portion 13that has a reverse mesa shape. If a crystal axis along which amesa-stripe is to be formed and etchant therefor can be selectedproperly, then the etching is carried out to form another shape of thesemiconductor ridge portion 13.

As shown in area (c) of FIG. 5, the etching mask 20 and protect layer 19are removed. As shown in area (d) of FIG. 5, the remaining portion ofthe second conductive type semiconductor region 7 and the contact layer17 are grown thereon (the second crystal growth). The electrodes 21 and23 are, finally, formed on the contact layer 17 and the backside of thesemiconductor substrate 11, respectively, to complete the semiconductoroptical device 1.

The technical advantages according to the present embodiment will bedescribed. In the optical semiconductor device according to the presentembodiment, the potential adjusting layer 2 is provided between theactive layer 5 and the second conductive type semiconductor region 7 andhas a bandgap energy different from bandgap energies of the firstconductive type semiconductor region 3 and the second conductive typesemiconductor region 7. The turn on voltage of the A portion in theoptical semiconductor device is changed by adjusting the bandgap energyof the potential adjusting layer 2. Since the series resistance is alsochanged thereby, the slope of the current vs. voltage characteristics inthe linear operation region is adjusted. Accordingly, the current vs.optical output characteristics of the optical semiconductor device 1 ischanged by controlling the bandgap energy of the potential adjustinglayer. Please note that another semiconductor optical device includinganother potential adjusting layer located between the active layer 5 andthe second portion 3 b of the first conductive type semiconductor region3 provides the same technical advantages as the semiconductor opticaldevice 1 according to the present embodiment.

One example of operations of the semiconductor optical device 1 will bedescribed. Curves G5 and G6 in areas (a) and (b) of FIG. 6 show therelationship between the applied voltage and current and therelationship between the applied current and optical output power,respectively. Areas (c) and (d) of FIG. 6 is a graph showing pulsedcurrent G7 applied to the semiconductor optical device 1 and a graphshowing optical output power G8 in response to the applied current,respectively.

If the output saturation in the current vs. voltage characteristics ofthe semiconductor optical device 1 is used, the cost of the auto powercontrol (hereinafter referred to as “APC”) circuit which keeps theoptical output from the directly modulated semiconductor laser diode tobe constant can be lowered due to the reasons mentioned below. In FIG.3, if the width of the B portion. (the width of the pn junctionconstituted by the first and second conductive type semiconductorregions) is much larger than that of the A portion (the width of thesemiconductor ridge 13), current flowing through the A portion after theturning-on of the B portion becomes constant because most of the currentapplied to the optical semiconductor device flows through the B portionafter the B portion is turned on. Therefore, as shown in area (b) inFIG. 6, when current greater than I_(B) is applied thereto, the opticalsemiconductor device 1 operates in the output saturation region and theoptical output is set to be a constant value P. Accordingly, ifmodulation current having a peak value I₂ greater than I_(B) is appliedto the optical semiconductor device, the peak value of the opticaloutput power from the optical semiconductor device 1 is kept constantwithout depending on the value I₂ of the applied current. What is neededto keep the peak value of the optical output power to be the constantvalue P is only that the peak value I₂ of the modulation current isgreater than I_(B). Thus, the APC is not needed to have a high accuracyin the control of the current which makes it possible to reduce the costof the APC. Accordingly, inexpensive APC circuits can be used in lightsource systems and this permits the cost down of light source systems.The peak value P of the modulated optical output can be changed widelyby use of the potential adjusting layer 2 in the present embodimentand/or other potential adjusting layers that will be described in thepresent specification. Accordingly, the peak value P can be set at adesired value appropriate to applications

The fabrication of the semiconductor optical device 1 can be simple, ascompared to the conventional one described in the publication 1. Buriedheterostructure type semiconductor lasers as in the publication 1 can befabricated as follows: after growing the active layer and etching thusgrown active layer by use of an etching mask, the current block portionis grown by use of the etching mask; then, the etching mask is removedand a p-type semiconductor layer is grown thereon. This fabricationneeds three steps of epitaxial growth. On the other hand, as seen fromthe foregoing explanations, the step of growing the current blockportion is not needed in the fabrication of the semiconductor opticaldevice according to the present embodiment. Therefore, the semiconductoroptical device permits the yield improvement and cost reduction becausethe number of the epitaxial growth steps is reduced (twice in thepresent embodiment).

In a modified structure of the semiconductor optical device 1, the sizeof the contact layer 17 is restricted and the restricted contact layeris located only on the part of the second conductive type semiconductor7 and the active layer 5. In the modified structure, the distancebetween the restricted contact layer 17 and the pn junction (the Bportion in FIG. 3) by the first and second conductive type semiconductorregions 3 and 7 is longer than the distance between the restrictedcontact layer 17 and the active layer 5. Accordingly, the resistancefrom the restricted contact layer 17 to the pn junction (the B portionin FIG. 3) is enhanced to increase the turn-on the voltage V_(B) of thepn junction (the B portion in FIG. 3). Consequently, the differencebetween the turn-on voltages V_(B) and V_(A), i.e., the difference,V_(B)−V_(A), is increased and the carriers are confined to the activelayer 5 even in a higher power region, thereby widening the linearoperation region.

In buried heterostructure semiconductor optical devices as shown in, forexample, Publication 1, the injected carriers are blocked using thecurrent blocking region having a pn junction, constituted by a p-typesemiconductor layer and an n-type semiconductor layer, which areinversely biased. However, in this type of lasers, a plurality of pnjunctions should be formed to realize a current blocking, which leads toa large parasitic capacitance, and prevents the buried heterostructuresemiconductor optical devices from operating at high-speed. On the otherhand, since the semiconductor optical device according to the presentembodiment blocks the injected carriers by use of the single pnjunction, which is biased forwardly, constituted by the first and secondconductive type semiconductor regions 3 and 7, only one pn junction isneeded for blocking current and thus the capacitance is decreasedcompared with buried heterostructure semiconductor optical devices as inPublication 1. Therefore, the semiconductor optical device 1 can operateat higher speed.

As described above, the surface of GaAs semiconductor can be provided byGaAs substrates. The surface of InP semiconductor can be provided by InPsubstrates. The surface of GaN semiconductor can be provided by GaNsubstrates. The surface of SiC semiconductor can be provided by SiCsubstrates. Since available GaAs substrates are large-sized and are highquality and inexpensive, the productivity improvement and cost reductionof the semiconductor optical device 1 are achieved and large-scaledintegration including the semiconductor optical device 1 can be realizedeasily.

Areas (a) and (b) of FIG. 7 show cross sectional views of a firstmodified semiconductor optical device according to the presentembodiment. As shown in area (a) of FIG. 7, a semiconductor opticaldevice 1 a includes a potential adjusting layer 2 a provided between thesecond region 3 d of the first conductive type semiconductor region 3and the second conductive type semiconductor region 7, instead ofbetween the active layer 5 and the second conductive type semiconductorregion 7. In this case, the first conductive type semiconductor region3, the bottom 7 b of second conductive type semiconductor region 7 andthe potential adjusting layer 2 a are arranged to form a pn junctiontherein. The second region 3 d of the first conductive typesemiconductor region 3 and the second conductive type semiconductorregion 7 (bottom 7 b) indirectly constitute a pn junction by use of thepotential adjusting layer 2 a provided therebetween. The firstconductive type semiconductor region 3, the second conductive typesemiconductor region 7 and the potential adjusting layer 2 a providedtherebetween are components of the pn junction. This pn junction has thefollowing configurations: if the potential adjusting layer 2 a has thefirst conductivity type, the potential adjusting layer 2 a and thesecond conductive type semiconductor region 7 constitute the pn junctionand the potential adjusting layer 2 a and the first conductive typesemiconductor region 3 constitute a pp-junction or nn-junction; if thepotential adjusting layer 2 a has the second conductivity type, thepotential adjusting layer 2 a and the first conductive typesemiconductor region 3 constitute the pn junction and the potentialadjusting layer 2 a and the second conductive type semiconductor region7 constitute an nn-junction or pp-junction; if the potential adjustinglayer 2 a is undoped, the potential adjusting layer 2 a and the firstconductive type semiconductor region 3 and the second conductive typesemiconductor region 7 constitute a pin junction.

When the bandgap energy of the potential adjusting layer 2 a is smallerthan the bandgap energies of the first and second conductive typesemiconductor regions 3 and 7, the resistance and the built-in potentialbetween the second region 3 d of the first conductive type semiconductorregion 3 and the second conductive type semiconductor region 7 becomesmaller as compared with the semiconductor optical device that does notinclude any potential adjusting layer at the relevant portion. Further,if the bandgap energy of the potential adjusting layer 2 a is largerthan the bandgap energies of the first and second conductive typesemiconductor regions 3 and 7, the resistance and the built-in potentialbetween the second region 3 d of the first conductive type semiconductorregion 3 and the second conductive type semiconductor region 7 becomegreater as compared with the semiconductor optical device that does notinclude any potential adjusting layer.

Ares (a) of FIG. 8 shows a graph representing the relationship betweenthe applied voltage and the applied current. Area (b) of FIG. 8 shows agraph representing the applied current and the optical output power forthe optical semiconductor device 1 a corresponding to the semiconductoroptical device 1 and curve G10 and G20 show the characteristicscorresponding to a semiconductor device without the potential adjustinglayer 2 a. If the potential adjusting layer 2 a has a bandgap energygreater than the bandgap energies of the second region 3 d of the firstconductive type semiconductor region 3 and the second conductive typesemiconductor region 7 and is provided therebetween (outside the activelayer), the resistance and the built-in potential between the secondregion 3 d of the first conductive type semiconductor region 3 and thesecond conductive type semiconductor region 7 become increased.Consequently, compared with a semiconductor optical device without apotential adjusting layer 2 a, the turn-on voltage V_(B0) in curve G10is increased to V_(B1) as shown in curve G13 in area (a) of FIG. 8 andthe turn-on voltage V_(A0) for the junction associated with the activelayer 5 remains unchanged because of no potential adjusting layer 2 a atthe relevant portion. Since the junction that does not include theactive layer 5 does not turn on below the current of I_(B1) that isgreater than I_(B0) in curve G10 (no potential adjusting layer 2 a), thesemiconductor optical device exhibits the current vs. optical outputpower characteristics in which the linear operation region becomes wideras shown in curve G23.

If the potential adjusting layer 2 a has a bandgap energy smaller thanthe bandgap energies of the second region 3 d of the first conductivetype semiconductor region 3 and the second conductive type semiconductorregion 7 and is provided therebetween, the resistance and the built-inpotential between the second region 3 d of the first conductive typesemiconductor region 3 and the second conductive type semiconductorregion 7 are lowered. Consequently, compared with a semiconductoroptical device without a potential adjusting layer 2 a, the turn-onvoltage V_(B0) of the junction by the first conductive typesemiconductor region 3 and the second conductive type semiconductorregion 7 are decreased to V_(B2) as shown in curve G14 in area (a) ofFIG. 8 and the turn-on voltage V_(A0) for the junction associated withthe active layer 5 remains unchanged because of no potential adjustinglayer 2 a at the relevant junction. Since the junction that does notinclude the active layer 5 does not turn on below the current of I_(B2)that is lower than I_(B0) in curve G10 (no potential adjusting layer 2a), the semiconductor optical device exhibits the current vs. opticaloutput power characteristics in which the optical output power issaturated in the lower power region thereof as shown in curve G24 inarea (b) of FIG. 8.

The current vs. optical output power characteristics of thesemiconductor optical device 1 a can be varied by selecting the bandgapenergy of the potential adjusting layer 2 a as with the case of thesemiconductor optical device 1.

Referring area (b) of FIG. 7, the semiconductor optical device 1 bincludes a potential adjusting layer 2 b and the potential adjustinglayer 2 b has a first region 22 a of the first conductive type and asecond region 22 b of the second conductive type. The first region 22 aof the potential adjusting layer 2 b and the first conductive typesemiconductor region 3 form a junction, and the second region 22 b ofthe potential adjusting layer 2 b and the second conductive typesemiconductor region 7 form a junction. The first and second regions 22a, 22 b of the potential adjusting layer 2 b forms a pn junction. If thebandgap energy of the potential adjusting layer 2 b has a bandgap energygreater than the bandgap energies of the first conductive typesemiconductor region 3 and the second conductive type semiconductorregion 7, the built-in potential of the pn junction of the first andsecond region 22 a and 22 b of the potential adjusting layer 2 b isgreater than the built-in potential of a direct junction by the firstconductive type semiconductor region 3 and the second conductive typesemiconductor region 7, and the resistance of the pn junction of thefirst and second region 22 a and 22 b is increased because thehetero-barrier at the pn junction by the first and second region 22 aand 22 b is increased. Accordingly, the turn on voltage V_(B) at this pnjunction is increased. As a result, the semiconductor optical deviceexhibits the current vs. optical output power characteristics in whichthe linear operation region becomes wider. On the other hand, if thepotential adjusting layer 2 b has a bandgap energy smaller than thebandgap energies of the first conductive type semiconductor region 3 andthe second conductive type semiconductor region 7, the built-inpotential of the pn junction constituted by the first and second region22 a and 22 b is smaller than the built-in potential of a directjunction by the first conductive type semiconductor region 3 and thesecond conductive type semiconductor region 7, and the resistance at thepn junction by the first and second region 22 a and 22 b is decreasedbecause the hetero-barrier at the pn junction by the first and secondregion 22 a and 22 b is decreased. Accordingly, the turn on voltageV_(B) at this pn junction is decreased. As a result, the semiconductoroptical device exhibits the current vs. optical output powercharacteristics in which the optical output power is saturated in thelower power region thereof. In this way, the current vs. optical outputpower characteristics can be varied by selecting the bandgap energy ofthe potential adjusting layer 2 b to adjust the built-in potentialbetween the first and second regions 22 a and 22 b.

FIG. 9 is a cross sectional view showing a second modified semiconductoroptical device according to the present invention. The modifiedsemiconductor optical device 1 c has a potential adjusting layer 2 cprovided in the following arrangement: between the active layer 5 andthe second conductive type semiconductor region 7; between the secondregion 3 d of the first conductive type semiconductor region 3 and thesecond conductive type semiconductor region 7. In this case, the secondregion 3 d of the first conductive type semiconductor region 3, thesecond conductive type semiconductor region 7 and the potentialadjusting layer 2 c are arranged to form a pn junction therein. Thesecond region 3 d of the first conductive type semiconductor region 3and the second conductive type semiconductor region 7 indirectlyconstitute a pn junction by use of the potential adjusting layer 2 cprovided therebetween. The second region 3 d of the first conductivetype semiconductor region 3, the second conductive type semiconductorregion 7 and the potential adjusting layer 2 c provided therebetween arecomponents of the pn junction. This pn junction has the followingconfigurations: if the potential adjusting layer 2 c has the firstconductivity type, the potential adjusting layer 2 c and the secondconductive type semiconductor region 7 constitute the pn junction andthe potential adjusting layer 2 c and the first conductive typesemiconductor region 3 constitute a pp-junction or nn-junction; if thepotential adjusting layer 2 c has the second conductivity type, thepotential adjusting layer 2 c and the first conductive typesemiconductor region 3 constitute the pn junction and the potentialadjusting layer 2 c and the second conductive type semiconductor region7 constitute an un-junction or pp-junction; if the potential adjustinglayer 2 c is undoped, the potential adjusting layer 2 c and the firstconductive type semiconductor region 3 and the second conductive typesemiconductor region 7 constitute a pin junction. The built-in potentialand resistance at one junction constituted by the active layer 5 and thesecond conductive type semiconductor region 7 and the built-in potentialand resistance at another junction constituted by the second region 3 dof the first conductive type semiconductor region 3 and the secondconductive type semiconductor region 7 are adjusted by changing thebandgap energy of the potential adjusting layer 2 c as described in theabove, and thus the current vs. optical output characteristics of thesemiconductor optical device 1 c can be varied.

FIGS. 10 to 12 are views showing third modified semiconductor opticaldevices 1 d to 1 f according he present embodiment. The modifiedsemiconductor optical devices 1 d to 1 f correspond to the semiconductoroptical device 1 and the modified semiconductor optical devices 1 a and1 c, respectively. The semiconductor optical devices 1 d to 1 f eachfurther comprises a first optical confinement layer 25 and a secondoptical confinement layer 27. The first optical confinement layer 25 isprovided between the first conductive type semiconductor region 3 andthe active layer 5. The second optical confinement layer 27 is providedbetween the second conductive type semiconductor region 7 and the activelayer 5. The second semiconductor portion 3 b of the first conductivetype semiconductor region 3, the active layer 5, the first opticalconfinement layer 25 and the second optical confinement layer 27constitute a semiconductor ridge portion 13 a.

The first optical confinement layer 25 is made of material having abandgap energy between that of the first conductive type semiconductorregion 3 and that of the active layer 5. The second optical confinementlayer 27 is made of material having a bandgap energy between that of thesecond conductive type semiconductor region 7 and that of the activelayer 5. Carriers are injected into the active layer 5 from the firstand second conductive type semiconductor regions 3 and 7 through thefirst and second optical confinement layers 25 and 27. The injectedcarriers in the modified semiconductor optical devices 1 d to 1 f areconfined into the active layer 5 by the first and second opticalconfinement layers 25 and 27.

The first optical confinement layer 25 has a refractive index betweenthat of the active layer 5 and that of the first conductive typesemiconductor region 3. The second optical confinement layer 27 has arefractive index between that of the active layer 5 and that of thesecond conductive type semiconductor region 7. The first conductive typesemiconductor region 3 and the second conductive type semiconductorregion 7 confine light from the active layer 5 into the first and secondoptical confinement layers 25 and 27 and the active layer 5.

The first and second optical confinement layers 25 and 27 permit thecurrent confinement and the optical confinement separately. Theseoptical confinement layers enhance the confinement of the light into theactive layer 5, leading to improvements of lasing characteristics suchas a threshold current, reduction and a less dependence on temperature.If the active layer 5 has a quantum well structure constituted by thinfilms, the optical confinement factor is small. But, by introducing thefirst and second optical confinement layers 25 and 27, the opticalconfinement factor of the quantum well structure increasessignificantly, thereby drastically improving the oscillationcharacteristics.

If a semiconductor optical device as shown in the above has the firstand second optical confinement layers 25 and 27, the potential adjustinglayer 2 in the semiconductor optical device 1 d can be provided in thefollowing arrangements: between the second optical confinement layer 27and the second conductive type semiconductor region 7 and/or between thefirst optical confinement layer 25 and the first conductive typesemiconductor region 3. The potential adjusting layer 2 a in thesemiconductor optical device 1 e can be provided between the secondconductive type semiconductor region 7 and the second region 3 d of thefirst conductive type semiconductor region 3 as with the first modifiedsemiconductor optical device. In the present modified semiconductoroptical device, the potential adjusting layer 2 b having the first andsecond regions 22 a and 22 b can be provided between the secondconductive type semiconductor region 7 and the second region 3 d of thefirst conductive type semiconductor region 3. The potential adjustinglayer 2 c in the semiconductor optical device 1 f can be provided in oneof the following arrangements: between the second conductive typesemiconductor region 7 and the second region 3 d of the first conductivetype semiconductor region 3; between the second optical confinementlayer 27 and the second conductive type semiconductor region 7; betweenthe second conductive type semiconductor region 7 and the second region3 d of the first conductive type semiconductor region 3 and between thefirst optical confinement layer 25 and the second semiconductor portion3 b of the first conductive type semiconductor region 3. The current vs.optical output power characteristics of the semiconductor opticaldevices 1 d to 1 f can be varied by the bandgap energies of thepotential adjusting layer 2, 2 a to 2 c.

Second Embodiment

FIG. 13 is a perspective view showing a semiconductor optical deviceaccording to the second embodiment. An XYZ coordinate system S isdepicted in FIG. 13. FIG. 14 is a cross sectional view taken alongIII-III in FIG. 13. FIGS. 13 and 14 shows a semiconductor optical device51, such as a semiconductor laser.

Referring to FIGS. 13 and 14, the semiconductor optical device 51comprises a potential adjusting layer 52, a first conductive typesemiconductor region 53, an active layer 55, and a second conductivetype semiconductor region 57. The first conductive type semiconductorregion 53 is provided on the surface of a GaAs semiconductor, InPsemiconductor, GaN semiconductor and SiC semiconductor, and has firstand second regions 53 a and 53 b. The second region 53 b is adjacent tothe first region 53 a. The first region 53 a extends in the z-direction.The active layer 55 is provided on the first region 53 a of the firstconductive type semiconductor region 53. The active layer 55 has a pairof sides 55 a. The second conductive type semiconductor region 57 (thebottom 57 b thereof) is provided on the second region 53 b of the firstconductive type semiconductor region 53, and the sides 55 a and top 55 bof the active layer 55. The potential adjusting layer 52 is providedbetween the active layer 55 and the second conductive type semiconductorregion 57. The potential adjusting layer 52 has a pair of sides 52 h.The second conductive type semiconductor region 57 and the second region53 b of the first conductive type semiconductor region 53 form a pnjunction around the active layer 55. The active layer 55 is made ofIII-V compound semiconductor.

The first conductive type semiconductor region 53 is made of III-Vcompound semiconductor, the bandgap of which is greater than that of theactive layer 55. The second conductive type semiconductor region 57 ismade of III-V compound semiconductor, the bandgap of which is greaterthan that of the active layer 55.

The potential adjusting layer 52 is made of III-V compoundsemiconductor, the bandgap energy of which has the following types. Inone of these types, the bandgap energy of III-V compound semiconductorof the potential adjusting layer 52 is greater than that of the activelayer 55 and is smaller than the bandgap energies of the first andsecond conductive type semiconductor regions 53 and 57. In the other,the bandgap energy of III-V compound semiconductor of the potentialadjusting layer 52 is greater than the bandgap energies of the first andsecond conductive type semiconductor regions 53 and 57.

The conductive type of the potential adjusting layer 52 is the same asthat of the second conductive type semiconductor region 57 or can beundoped as is the case with the active layer 55. If the conductivitytype of the active layer 55 has p-type or n-type, then the conductivetype of the potential adjusting layer 52 is the same as that of theactive layer.

The first conductive type semiconductor region 53 and second conductivetype semiconductor region 57 confine carriers to the active layer 55.Consequently, the first conductive type semiconductor region 53 works asa cladding layer of the first conductive type and the second conductivetype semiconductor region 57 works as a cladding layer of the secondconductive type. In the active layer 55, the confined carriers injectedfrom the first conductive type semiconductor region 53 and the secondconductive type semiconductor region 57 are recombined to generatelight.

The refractive index of the first conductive type semiconductor region53 is smaller than that of active layer 55. The refractive index of thesecond conductive type semiconductor region 57 is also smaller than thatof active layer 55. Accordingly, the first conductive type semiconductorregion 53 and the second conductive type semiconductor region 57 confinelight from the active layer 55 into the active layer 55 in both x and ydirections. Consequently, the first conductive type semiconductor region53 and the second conductive type semiconductor region 57 act as opticalcladding layers.

The structure of the active layer 55 may be the bulk structure of asingle layer, the single quantum well structure of a single quantum welllayer or the multiple quantum well structure of a plurality of welllayers and barrier layers which are alternately arranged.

The semiconductor optical device 51 further comprises a semiconductorsubstrate 61. For example, GaAs substrates, InP substrates, GaNsubstrates and SiC substrates can be used as the semiconductor substrate61. These substrates can provide the surface of GaAs semiconductor, InPsemiconductor, GaN semiconductor and SiC semiconductor on which thefirst conductive type semiconductor region 53 is grown.

The semiconductor optical device 51 further comprises a contact layer 67of the second conductive type, and electrodes 71 and 73. The contactlayer 67 is provided on the second conductive type semiconductor region57. The electrode 71 is provided on the contact layer 67. The electrode71 extends in a direction in which the active layer 55 extends. Theelectrode 73 is provided on the backside 61 b of the semiconductorsubstrate 61. The bandgap of the contact layer 67 is smaller than thatof the second conductive type semiconductor layer 57. Accordingly, thecontact layer 67 and the electrode 71 can form an excellent ohmiccontact therebetween.

In the semiconductor optical device 51, since the first conductive typesemiconductor region 53 and the second conductive type semiconductorregion 57 each has a bandgap energy greater than that of the bandgap ofthe active layer 55, the built-in potential of the pn junctionconstituted by the first conductive type semiconductor region 53 and thesecond conductive type semiconductor region 57 is greater than that ofthe junction constituted by the first conductive type semiconductorregion 53, the active layer 55 and the second conductive typesemiconductor region 57. Therefore, the semiconductor optical device 51has the same circuit as in FIG. 3 and operates in the same manner as thesemiconductor optical device 1. Namely, carriers in the first conductivetype semiconductor region 53 and the second conductive typesemiconductor region 57 are blocked by the pn junction constituted bythe first conductive type semiconductor region 53 and the secondconductive type semiconductor region 57, and are exclusively injectedand confined into the active layer 55. Thus, the semiconductor opticaldevice 51 is effective in confining the carriers into the active layer55.

If the bandgap energy of the potential adjusting layer 52 is smallerthan the bandgap energies of the first conductive type semiconductorregion 53 and the second conductive type semiconductor region 57 and isgreater than that of the active layer 55, then the hetero-barrierbetween the second conductive type semiconductor region 57 and theactive layer 55 is reduced and the resistance thereat is lowered.Consequently, the turn-on voltage V_(A) of the junction constituted bythe first conductive type semiconductor region 53, the second conductivetype semiconductor region 57 and the active layer 55 is lowered, and theslope of the current vs. voltage characteristics after the turn-on ofthe A portion (shown in FIG. 3), i.e., the series resistance is deceasedin comparison with the semiconductor optical device without a potentialadjusting layer 52. Therefore, the current I_(B) corresponding to theturn-on voltage V_(B) of the pn junction constituted by the secondregion 53 b of the first conductive type semiconductor region 53 and thesecond conductive type semiconductor region 57 becomes larger than thatof a semiconductor optical device without a potential adjusting layer52. As a result, since the B portion (shown in FIG. 3) of thesemiconductor optical device 51 does not turn on up to a largerinjection current, the semiconductor optical device 51 has a current vs.optical power relationship in which the width of the linear operationregion is enlarged, thereby increasing the optical power in the linearoperation region. Furthermore, if the bandgap energy of the potentialadjusting layer 52 is greater than the bandgap energies of the first andsecond conductive type semiconductor regions 53 and 57, then thehetero-barrier between the active layer 55 and the second conductivetype semiconductor region 57 is increased and the resistance thereofbecomes greater. Therefore, the turn on voltage V_(A) is increased, andthe slope in the current vs. voltage characteristics after the turn-onvoltage of the A portion, i.e., series resistance, is increased.Therefore, the current I_(B) corresponding to the turn-on voltage V_(B)of the pn junction constituted by the second region 53 b of the firstconductive type semiconductor region 53 and the second conductive typesemiconductor region 57 becomes smaller than that of a semiconductoroptical device without a potential adjusting layer. As a result, sincethe B portion (shown in FIG. 3) of the semiconductor optical device 51does not turn on up to a larger injection current, the optical outputpower is saturated in a lower output region in the current vs. opticaloutput power characteristics. As described above, the current vs.optical output power characteristics of the semiconductor optical device51 can be varied by selecting the bandgap energy of the potentialadjusting layer 52. Although the potential adjusting layer 52 is locatedbetween the active layer 55 and the second conductive type semiconductorregion 57, the potential adjusting layer 52 can be located between theactive layer 55 and the first conductive type semiconductor region 53,thereby providing the same technical advantages.

The method of fabricating the semiconductor optical device 51 isdifferent from the method of fabricating the semiconductor opticaldevice 1 (FIG. 5) in the following: the method of fabricating thesemiconductor optical device 51 does not include the etching of thefirst conductive type semiconductor region 53 in the etching processshown in area (b) of FIG. 5. This method does not include the growth ofcurrent block portion and thus the number of epitaxial growth steps isdecreased as with the case of the semiconductor optical device 1.

In a modified structure of the semiconductor optical device 51, the sizeof the contact layer 67 can be restricted and the restricted contactlayer is located only on a part of the second conductive typesemiconductor region 57 and above the active layer 55. In the modifiedstructure, the distance between the restricted contact layer 67 and thepn junction (the B portion in FIG. 3) constituted by the first andsecond conductive type semiconductor regions 53 and 57 is longer thanthe distance between the restricted contact layer 67 and the activelayer 55. Accordingly, the resistance from the restricted contact layer67 to the pn junction (the B portion in FIG. 3) is enhanced to increasethe turn-on voltage V_(B) of the pn junction (the B portion in FIG. 3).Consequently, the difference V_(B)−V_(A) between the turn-on voltagesV_(B) and V_(A) is increased and the carriers are confined to the activelayer 55 even in a higher power region, thereby widening the linearoperation region.

Since the semiconductor optical device 51 according to the presentembodiment blocks the injected carriers by use of the single pn junctionwhich is biased forwardly and is constituted by the first and secondconductive type semiconductor regions 53 and 57, only one pn junction isneeded for blocking current and thus the capacitance is decreased, ascompared with conventional buried heterostructure semiconductor opticaldevices as in Publication 1. Therefore, the semiconductor optical device51 can operate at high speed.

As described above, the surface of GaAs semiconductor can be provided byGaAs substrates. Since available GaAs substrates are large-sized such as6 inch in a diameter and are high quality and inexpensive, theproductivity improvement and cost reduction of the semiconductor opticaldevice 51 are achieved and large-scaled integration of the semiconductoroptical device 51 can be easily realized.

In the present embodiment, the first conductive type semiconductorregion 53 can be made of material resistant to an etchant for etchingthe active layer 55 and the potential adjusting layer 52, and functionsas a etching stopper therefor. In conventional buried hetero-structuresas in publication 1, etching the active layer into a mesa-shape iscarried out using wet etching in most cases to avoid the damage ofsemiconductor portions during the etching process. Since wet etching is,however, isotropic, the etchant etches the active layer in bothhorizontal and vertical directions. Consequently, the width of theactive layer is varied depending on the mesa depth. For example, in thefabrication of the semiconductor laser device as described inPublication 1, etchant of Br-methanol is generally used to etch theactive layer made of GaInAsP. But, because the InP layer is also etchedby the etchant of Br-methanol, this etchant can etch not only the activelayer but also the n-type InP cladding layer located just below theactive layer. Etching rates in wet etchings are varied depending on evenslight fluctuations of the etchant temperature, the etchantconcentration and the mixture ratios of etchant. Especially, Br-methanolis volatile and thus the etching rate thereof is easily varied. Inaddition, etching rates on the wafer cannot be constant all over thesurface of the wafer due to the difference of stirring speed of theetchant between the center and the periphery of the wafer. Due to thisvariation of etching rate, the mesa depth varies in every production andall over the surface of the wafer. Consequently, the width of the activelayer is also varied. Accordingly, precise control of the width of theactive layer is difficult, which would affect the reproducibility anduniformity of laser characteristics.

On the other hand, since the semiconductor optical device 51 accordingto the present embodiment uses the GaAs substrate, AlGaInP or GaInP canbe used for the first conductive type semiconductor region 53, AlGaAs,GaAs, GaInAsP or GaInAs can be used for the potential adjusting layer52, and AlGaAs, GaAs, GaInAsP, GaInAs and III-V compound semiconductorcontaining at least nitrogen, gallium and arsenic can be used for theactive layer 55. In this case, the first conductive type semiconductorregion 53 works as an etching stopper in the etching of the active layer55 and the potential adjusting layer 52 by use of appropriate etchant(for example, phosphoric-acid-based etchant), whereby the active layer55 and the potential adjusting layer 52 are etched without the etchingof the first conductive type semiconductor region 53 located just below.As a result, the excellent reproducibility and uniformity of the mesadepth of the active layer 55 and the potential adjusting layer 52 areobtained and accordingly the better reproducibility and uniformity ofthe width of the active layer 55 and the potential adjusting layer 52are obtained, thereby improving the reproducibility and uniformity oflaser characteristics.

Areas (a) and (b) of FIG. 15 show cross sectional views of a fourthmodified semiconductor optical device according to the presentembodiment. As shown in area (a) of FIG. 15, a semiconductor opticaldevice 51 a includes a potential adjusting layer 52 a provided betweenthe second region 53 b of the first conductive type semiconductor region53 and the second conductive type semiconductor region 57, instead ofbetween the active layer 55 and the second conductive type semiconductorregion 57. In this device, the first conductive type semiconductorregion 53, the second conductive type semiconductor region 57 (thebottom 57 b thereof) and the potential adjusting layer 52 a are arrangedto form a pn-junction therein. The second region 53 b of firstconductive type semiconductor region 53 and the second conductive typesemiconductor region 57 indirectly constitute a pn junction by use ofthe potential adjusting layer 52 a provided therebetween. The firstconductive type semiconductor region 53, the second conductive typesemiconductor region 57 and the potential adjusting layer 52 a providedtherebetween are components of the pn junction. This pn junction has thefollowing configurations: if the potential adjusting layer 52 a has thefirst conductive type, the potential adjusting layer 52 a and the secondconductive type semiconductor region 57 constitute the pn junction andthe potential adjusting layer 52 a and the first conductive typesemiconductor region 53 constitute a pp-junction or nn-junction; if thepotential adjusting layer 52 a has the second conductive type, thepotential adjusting layer 52 a and the first conductive typesemiconductor region 53 constitute the pn junction and the potentialadjusting layer 52 a and the second conductive type semiconductor region57 constitute a pp-junction or nn-junction; if the potential adjustinglayer 52 a is undoped, the potential adjusting layer 52 a, the firstconductive type semiconductor region 53 and the second conductive typesemiconductor region 57 constitute a pin junction.

When the bandgap energy of the potential adjusting layer 52 a is smallerthan the bandgap energies of the first and second conductive typesemiconductor regions 53 and 57, the resistance and the built-inpotential between the second region 53 b of the first conductive typesemiconductor region 53 and the second conductive type semiconductorregion 57 become smaller as compared with the semiconductor opticaldevice that does not include any potential adjusting layer 52 a.Furthermore, if the bandgap energy of the potential adjusting layer 52 ais larger than the bandgap energies of the first and second conductivetype semiconductor regions 53 and 57, the resistance and the built-inpotential between the second region 53 b of the first conductive typesemiconductor region 53 and the second conductive type semiconductorregion 57 become greater as compared with the semiconductor opticaldevice that does not include any potential adjusting layer 52 a.

If the potential adjusting layer 52 a has a bandgap energy greater thanthe bandgap energies of the second region 53 b of the first conductivetype semiconductor region 53 and the second conductive typesemiconductor region 57 and is provided therebetween (outside the activelayer), the resistance and the built-in potential between the secondregion 53 b of the first conductive type semiconductor region 53 and thesecond conductive type semiconductor region 57 are increased.Consequently, the turn-on voltage V_(B) are increased, while the turn-onvoltage V_(A) for the junction that does not include any potentialadjusting layer remains unchanged. As a result, the current I_(B)corresponding to the turn-on voltage V_(B) of the pn junctionconstituted by the second region 53 b of the first conductive typesemiconductor region 53, the potential adjusting layer 52 a, and thesecond conductive type semiconductor region 57 becomes larger than thatof a semiconductor optical device without any potential adjusting layer52 a. Consequently, the semiconductor optical device 51 a exhibits thecurrent vs. optical output power characteristics in which the linearoperation region becomes wider.

If the potential adjusting layer 52 a has a bandgap energy smaller thanthe bandgap energies of the second region 53 b of the first conductivetype semiconductor region 53 and the second conductive typesemiconductor region 57 and is provided therebetween, the resistance andthe built-in potential between the second region 53 b of the firstconductive type semiconductor region 53 and the second conductive typesemiconductor region 57 are lowered. Consequently, the turn-on voltageV_(B) are decreased, while the turn-on voltage V_(A) for the junctionthat does not include potential adjusting layer 52 a remains unchanged.As a result, the pn junction constituted by the following: the firstconductive type semiconductor layer 53, the potential adjusting layer 52a, and the second conductive type semiconductor region 57 becomessmaller than that of a semiconductor optical device without anypotential adjusting layer 52 a. Consequently, the semiconductor opticaldevice 51 a exhibits the current vs. optical output powercharacteristics in which the optical output power is saturated in thelower output region thereof.

Thus, the current vs. optical output power characteristics of thesemiconductor optical device 51 a can be varied by selecting thepotential adjusted layer 52 a as with the case of the semiconductoroptical device 51.

Referring area (b) of FIG. 15, the semiconductor optical device 51 bincludes a potential adjusting layer 52 b and the potential adjustinglayer 52 b has a first region 72 a of the first conductive type and asecond region 72 b of the second conductive type. The first region 72 aof the potential adjusting layer 52 b and the first conductive typesemiconductor region 53 form a junction, and the second region 72 b ofthe potential adjusting layer 52 b and the second conductive typesemiconductor region 57 form a junction. The first and second regions 72a, 72 b of the potential adjusting layer 52 b form a pn junction. If thebandgap energy of the potential adjusting layer 52 b has a bandgapenergy greater than the bandgap energies of the first conductive typesemiconductor region 53 and the second conductive type semiconductorregion 57, the built-in potential of the pn junction of the first andsecond regions 72 a and 72 b of the potential adjusting layer 52 b isgreater than the built-in potential of the direct junction between thefirst conductive type semiconductor region 53 and the second conductivetype semiconductor region 57, thereby increasing the turn on voltageV_(B) at this pn junction compared with the semiconductor optical devicewithout the potential adjusting layer 52 b. Therefore, the semiconductoroptical device exhibits the current vs. optical output powercharacteristics in which the linear operation region becomes wider andlinear operation can be maintained up to a higher output region. On theother hand, if the potential adjusting layer 52 b has a bandgap energysmaller than the bandgap energies of the first conductive typesemiconductor region 53 and the second conductive type semiconductorregion 57, the built-in potential of the pn junction of the first andsecond region 72 a and 72 b of the potential adjusting layer 52 b issmaller than the built-in potential of the direct junction between thefirst conductive type semiconductor region 53 and the second conductivetype semiconductor region 57, thereby decreasing the turn on voltageV_(B) at this pn junction compared with the semiconductor optical devicewithout the potential adjusting layer 52 b. Therefore, the semiconductoroptical device exhibits the current vs. optical output powercharacteristics in which the optical output power is saturated in thelower output power region thereof. As described above, the current vs.optical output characteristics can be varied by selecting the potentialadjusting layer 52 b to adjust the built-in potential at the junctionconstituted by the first and second region 72 a and 72 b.

FIG. 16 is a cross sectional view showing a fifth modified semiconductoroptical device according to the present invention. The modifiedsemiconductor optical device 51 c comprises a potential adjusting layer52 c provided in the following arrangement: between the active layer 55and the second conductive type semiconductor region 57; between thesecond region 53 b of the first conductive type semiconductor region 53and the second conductive type semiconductor region 57. In this case,the second region 53 b of the first conductive type semiconductor region53, the second conductive type semiconductor region 57 and the potentialadjusting layer 52 c are arranged to form a pn junction therein. Thesecond region 53 b of the first conductive type semiconductor region 53and the second conductive type semiconductor region 57 indirectlyconstitute a pn junction by use of the potential adjusting layer 52 cprovided therebetween. The second region 53 b of the first conductivetype semiconductor region 53, the second conductive type semiconductorregion 57 and the potential adjusting layer 52 c provided therebetweenare components of the pn junction. This pn junction has the followingconfigurations: if the potential adjusting layer 52 c has the secondconductivity type, the potential adjusting layer 52 c and the firstconductive type semiconductor region 53 constitute the pn junction andthe potential adjusting layer 52 c and the second conductive typesemiconductor region 57 constitute a pp-junction or nn-junction; if thepotential adjusting layer 52 c has the first conductivity type, thepotential adjusting layer 52 c and the second conductive typesemiconductor region 57 constitute the pn junction and the potentialadjusting layer 52 c and the first conductive type semiconductor region53 constitute an nn-junction or pp-junction; if the potential adjustinglayer 52 c is undoped, the potential adjusting layer 52 c and the firstconductive type semiconductor region 53 and the second conductive typesemiconductor region 57 constitute a pin junction. Since the built-inpotential and resistance between the active layer 55 and the secondconductive type semiconductor region 57 and the built-in potential andresistance between the second region 53 b of the first conductive typesemiconductor region 53 and the second conductive type semiconductorregion 57 are adjusted by changing the bandgap energy of the potentialadjusting layer 52 c as described in the above, the current vs. opticaloutput characteristics of the semiconductor optical device 51 c can bechanged.

In the semiconductor optical devices 51 a, 51 b and 51 c, the firstconductive type semiconductor region 53 can be made of materialresistant to an etchant for etching active layer 55 and that functionsas a etching stopper therefor. In this case, due to the excellentreproducibility and uniformity of the mesa depth of the active layer 55,the better reproducibility and uniformity of the width of the activelayer 55 are obtained, which leads to improving the reproducibility anduniformity of laser characteristics. Since the semiconductor opticaldevice 51 a, 51 b, and 51 c according to the present embodiment uses theGaAs substrate, AlGaInP or GaInP can be used for the first conductivetype semiconductor region 53, AlGaAs, GaAs, GaInAsP, GaInAs and III-Vcompound semiconductor containing at least nitrogen, gallium and arseniccan be used for the active layer 55. In this case, the first conductivetype semiconductor region 53 works as an etching stopper in the etchingof the active layer 55 by use of appropriate etchant (for example,phosphoric-acid-based etchant), whereby the active layer 55 is etchedwithout etching the first conductive type semiconductor region 53.

FIG. 17 is a cross sectional view showing a sixth modified semiconductoroptical device according to the present invention. The modifiedsemiconductor optical device 51 d comprises a potential adjusting layer52 d provided in the following arrangement: between the first region 53a of the first conductive type semiconductor region 53 and the activelayer 55; between the second region 53 b of the first conductive typesemiconductor region 53 and the second conductive type semiconductorregion 57. In this case, the surface of potential adjusting layer 52 dis planar. Since the built-in potential and resistance between the firstregion 53 a of the first conductive type semiconductor region 53 and theactive layer 55 and the built-in potential and resistance between thesecond region 53 b of the first conductive type semiconductor region 53and the second conductive type semiconductor region 57 are adjusted byselecting the bandgap energy of the potential adjusting layer 52 d asdescribed in the above, the current vs. optical output characteristicsof the semiconductor optical device 51 d can be varied.

In the semiconductor optical device 51 d, the potential adjusting layer52 d can be made of material resistant to an etchant for etching activelayer 55 and functions as a etching stopper therefor. In this case, dueto the excellent reproducibility and uniformity of the mesa depth of theactive layer 55, the better reproducibility and uniformity of the widthof the active layer 55 are obtained, which leads to improving thereproducibility and uniformity of laser characteristics. Since thesemiconductor optical device 51 d according to the present embodimentuses the GaAs substrate, AlGaInP or GaInP can be used for the potentialadjusting layer 52 d, AlGaAs, GaAs, GaInAsP, GaInAs and III-V compoundsemiconductor containing at least nitrogen, gallium and arsenic can beused for the active layer 55. In this case, the potential adjustinglayer 52 d works as an etching stopper in the etching of the activelayer 55 by use of appropriate etchant (for example,phosphoric-acid-based etchant).

FIG. 18 is a cross sectional view showing a seventh modifiedsemiconductor optical device according to the present invention. Themodified semiconductor optical device 51 e comprises a potentialadjusting layer 52 e provided in the following arrangement: between thefirst region 53 a of the first conductive type semiconductor region 53and the active layer 55; between the second region 53 b of the firstconductive type semiconductor region 53 and the second conductive typesemiconductor region 57. In this case, the potential adjusting layer 52e has a ridge portion 52 f provided on the first region 53 a of thefirst conductive type semiconductor region 53. The ridge portion 52 fhas a pair of sides 52 g and extends in the z direction. Since thebuilt-in potential and resistance between the first region 53 a of thefirst conductive type semiconductor region 53 and the active layer 55and the built-in potential and resistance between the second region 53 bof the first conductive type semiconductor region 53 and the secondconductive type semiconductor region 57 are adjusted by selecting thebandgap energy of the potential adjusting layer 52 e as described in theabove, the current vs. optical output characteristics of thesemiconductor optical device 51 e can be varied.

FIGS. 19 to 23 are views showing eighth modified semiconductor opticaldevices according the present embodiment. The modified semiconductoroptical devices 51 f to 51 j correspond to the semiconductor opticaldevice 51 and the modified semiconductor optical devices 51 a, 51 c, 51d and 51 e, respectively. The semiconductor optical devices 51 f to 51 jeach further comprises a first optical confinement layer 75 and a secondoptical confinement layer 77. The first optical confinement layer 75 isprovided between the first conductive type semiconductor region 53 andthe active layer 55. The second optical confinement layer 77 is providedbetween the second conductive type semiconductor region 57 and theactive layer 55. The first optical confinement layer 75 and secondoptical confinement layer 77 have the same configurations and functionsas the first optical confinement layer 25 and second optical confinementlayer 27, respectively. That is, the first optical confinement layer 75is made of material having a bandgap energy between that of the firstconductive type semiconductor region 53 and that of the active layer 55.The second optical confinement layer 77 is made of material having abandgap energy between that of the second conductive type semiconductorregion 57 and that of the active layer 55. The first optical confinementlayer 75 has a refractive index between that of the active layer 55 andthat of the first conductive type semiconductor region 53. The secondoptical confinement layer 77 has a refractive index between that of theactive layer 55 and that of the second conductive type semiconductorregion 57. The first and second optical confinement layers 75 and 77permit the current confinement and the optical confinement separately.These optical confinement layers enhance the confinement of the lightinto the active layer 55, leading to improvements of lasingcharacteristics such as a threshold current, reduction and a lessdependence on temperature.

If a semiconductor optical device as shown in the above has the firstand second optical confinement layers 75 and 77, the potential adjustinglayer 52 in the semiconductor optical device 51 f can be provided in thefollowing arrangements: between the second optical confinement layer 77and the second conductive type semiconductor region 57 and/or betweenthe first optical confinement layer 75 and the first conductive typesemiconductor region 53. Furthermore, the potential adjusting layer 52 ain the semiconductor optical device 51 g can be provided between thesecond conductive type semiconductor region 57 and the second region 53b of the first conductive type semiconductor region 53. In the presentmodification, the potential adjusting layer 52 b having the first andsecond regions 72 a and 72 b can be provided between the secondconductive type semiconductor region 57 and the second region 53 b ofthe first conductive type semiconductor region 53. The potentialadjusting layer 52 c in the semiconductor optical device 51 h can beprovided in the following arrangement: between the second conductivetype semiconductor region 57 and the second region 53 b of the firstconductive type semiconductor region 53; between the second opticalconfinement layer 77 and the second conductive type semiconductor region57. The potential adjusting layers 52 d and 52 e of the semiconductoroptical devices 51 i and 51 j are provided between the second region 53b of the first conductive type semiconductor region 53 and the secondconductive type semiconductor region 57 and between the first opticalconfinements layer 75 and the first region 53 a of the first conductivetype semiconductor region 53. The current vs. optical output powercharacteristics of the semiconductor optical devices 51 f to 51 j can bechanged by the bandgap energies of the potential adjusting layer 52, 52a to 52 e.

In the semiconductor optical device 51 f, the first conductive typesemiconductor region 53 can be made of material that functions as aetching stopper for etching active layer 55, the potential adjustinglayer 52 and the first and second optical confinement layers 75 and 77.In the semiconductor optical devices 51 g and 51 h, the first conductivetype semiconductor region 53 can be made of material resistant to anetchant for etching active layer 55 and the first and second opticalconfinement layers 75 and 77 and functions as a etching stoppertherefor. In this case, due to the excellent reproducibility anduniformity of the mesa depth of the active layer 55, the betterreproducibility and uniformity of the width of the active layer 55 areobtained, which leads to improving the reproducibility and uniformity oflaser characteristics. When the semiconductor optical device 51 f, 59 gand 51 h according to the present embodiment uses the GaAs substrate,AlGaInP or GaInP can be used for the first conductive type semiconductorregion 53, AlGaAs, GaAs, GaInAsP or GaInAs can be used for the potentialadjusting layer 52, AlGaAs, GaAs or GaInAsP can be used for the opticalconfinement layers 75 and 77, and AlGaAs, GaAs, GaInAsP, GaInAs andIII-V compound semiconductor containing at least nitrogen, gallium andarsenic can be used for the active layer 55. The first conductive typesemiconductor region 53 works as an etching stopper in the etching ofthe above layers by use of appropriate etchant (for example,phosphoric-acid-based etchant).

In the semiconductor optical device 51 i, the potential adjusting layer52 d can be made of material resistant to an etchant for etching activelayer 55 and the first and second optical confinement layers 75 and 77and functions as a etching stopper therefor. In this case, due to theexcellent reproducibility and uniformity of the mesa depth of the activelayer 55, the better reproducibility and uniformity of the width of theactive layer 55 are obtained, which leads to improving thereproducibility and uniformity of laser characteristics. Since thesemiconductor optical device 51 i according to the present embodimentuses the GaAs substrate as the substrate 61, AlGaInP or GaInP can beused for the potential adjusting layer 52 d, AlGaAs, GaAs and GaInAsPcan be used for the first and second optical confinement layers 75 and77, AlGaAs, GaAs, GaInAsP, GaInAs and III-V compound semiconductorcontaining at least nitrogen, gallium and arsenic can be used for theactive layer 55. In this case, the potential adjusting layer 52 d worksas an etching stopper in the etching of the active layer 55 and firstand second optical confinement layers 75 and 77 by use of appropriateetchant (for example, phosphoric-acid-based etchant).

FIG. 24 is a cross sectional view showing the ninth modifiedsemiconductor optical device according to the present embodiment. In themodified semiconductor optical device 51 k, a first conductive typesemiconductor region 54 includes a concentration changing region 54 aand another region 54 b and a second conductive type semiconductorregion 58 includes a concentration changing region 58 a and anotherregion 58 b. The concentration changing region 54 a of the firstconductive type semiconductor region 54 and the second conductive typesemiconductor region 58 forms an interface 54 c. The concentrationchanging region 58 a of the second conductive type semiconductor region58 and the first conductive type semiconductor region 54 formsinterfaces 58 c and 58 d. In the first conductive type semiconductorregion 54, the dopant concentration of the concentration changing region54 a is different from that of the other region 54 b, and in the secondconductive type semiconductor region 58, the dopant concentration of theconcentration changing region 58 a is different from that of the otherregion 58 b.

Area (a) of FIG. 25 is a graph showing curve G16 for representing thecurrent vs. voltage characteristics of the modified semiconductoroptical device 51 k and showing curve G15 for representing the currentvs. voltage characteristics of a semiconductor optical device withoutthe concentration changing regions 54 a and 58 a (as is the case withthe semiconductor optical device 51) which is different from themodified semiconductor optical device 51 k. Area (b) of FIG. 25 is agraph showing curves G25 and G26 in the current vs. optical outputcharacteristics that corresponds to curves G15 and G16, respectively.The concentration changing region 54 a has a dopant concentrationdifferent form the other region 54 b and thus the quasi-Fermi level andresistance of the concentration changing region 54 a is changed bychanging the dopant concentration in this region. The concentrationchanging region 58 a has a dopant concentration different form the otherregion 58 b and thus the quasi-Fermi level and resistance of theconcentration changing region 58 a is changed by changing the dopantconcentration in this region. Accordingly, the turn-on voltages at thepn junction constituted by the first conductive type semiconductorregion 54 and second conductive type semiconductor region 58 and at thepin junction constituted by the first conductive type semiconductorregion 54, the active layer 55, the potential adjusting layer 52 andsecond conductive type semiconductor region 58 are changed, and theseries resistance of the modified semiconductor optical device in thelinear operation region after the turning-on the pin junction ischanged. For example, if the dopant concentration of the concentrationchanging region 54 a is greater than that of the other region 54 b andthe dopant concentration of the concentration changing region 58 a isgreater than that of the other region 58 b, the resistance values ofthese cladding regions are lowered and thus the turn-on voltages of thepn junction and pin junction are decreased. Therefore, the turn-onvoltage V_(A3) of the pin junction is lowered to the voltage V_(A4) andthe turn-on voltage V_(B3) of the pn junction is lowered to the voltageV_(B4). Furthermore, since the resistances of the concentration changingregion 54 a and the concentration changing region 58 a are alsodecreased, the slope (series resistance) of the current vs. voltagecharacteristics in the linear operation region after turning-on of thepin junction is lowered in the linear operation region. As a result, thecurrent flowing through the pn junction outside the active layer at theturned-on voltage of the pn junction is increased from I_(B3) to I_(B4)and thus the linear operation region of the current vs. optical outputcharacteristics becomes wider, thereby providing larger optical outputpower in the linear operation region. As described above, bydifferentiating the dopant concentrations of the concentration changingregions 54 a and 58 a from the dopant concentrations of the otherregions 54 b and 58 b, the turn-on voltage of the current vs. voltagecharacteristics and the series resistance after turning on the pinjunction are changed, so that the width of the linear operation regioncan be changed according to the intended use. In the above description,although the modified semiconductor optical device has bothconcentration changing regions 54 a and 58 a, only one of theconcentration changing regions 54 a and 58 a can be used for themodified semiconductor optical device and provides the same technicaladvantages as mentioned in the semiconductor optical device 51 k.

The turn-on voltages as above can be also adjusted by the change of thedopant concentration of the entire first conductive type semiconductorregion 54 (and/or the entire second conductive type semiconductor region58). Besides, the turn-on voltages can be also adjusted by changing thedopant concentration of only one of the first conductive typesemiconductor region 54 and the second conductive type semiconductorregion 58. In the above example of the semiconductor optical device 51k, dopant concentration changes are performed in only necessary parts ofthe first conductive type semiconductor region 54 and the secondconductive type semiconductor region 58. This is preferable to minimizethe degradation of other device characteristics caused by the dopantconcentration change. This dopant concentration change is not onlyapplicable to the semiconductor optical device 51, but it is applicableto the semiconductor optical device 1 in the first embodiment, thesemiconductor optical device 51 b in the present embodiment and othersemiconductor optical device according to the present invention.

In the semiconductor optical device according to the preferredembodiments and their modifications described above, the potentialadjusting layer having the thickness of less than several tens ofnanometers (for example, 5 nm) has the same technical contributions asmentioned above. Such a rhin potential adjusting layer is preferable tominimizing its effect on device characteristics other than current vs.voltage characteristics. However, the thickness of the potentialadjusting layer can be increased if it is necessary for the intendeduse.

The potential adjusting layer may have a composition such that thelattice mismatch between the potential adjusting layer and the substrateor base layer is from −3% to +3%. Since the thickness of the potentialadjusting layer can be very thin and is thinner than the criticalthickness, the above range of lattice mismatch does not generate crystaldefects such as misfit dislocation, and a good crystalline quality canbe maintained. In this case, since the restriction on the lattice matchcondition between the potential adjusting layer and the base layer isalleviated, the potential adjusting layer can be made of a wider rangeof materials. Accordingly, the bandgap energy of the potential adjustinglayer can be changed more widely, leading to more flexibility indesigning the semiconductor optical devices. If GaAs substrates areused, examples of the potential adjusting layer are as follows: AlGaInP,GaInP, GaInAsP, GaInAs or the like. If InP substrates are used, examplesof the potential adjusting layer are as follows: GaInAsP, GaInAs,AlGaInAs or the like. Preferably, the thickness of the strainedpotential adjusting layer is in the range of 5 nm to 10 nm, and thethickness of about 5 nm is more preferable.

A number of combinations of semiconductor materials that can provide theadvantages of the present invention will be explained. FIG. 26 is a viewshowing the list for the combinations of semiconductor materials. Thesecombinations of materials can be used for the semiconductor opticaldevices according to the embodiments and the modified embodiments.

In the semiconductor optical device according to the embodiments, GaAscan be used as the material of the semiconductor substrate.Alternatively, the GaAs surface can be formed by growing a GaAs layer ona substrate of material different from GaAs. Further, it is preferableto use III-V semiconductor material containing at least nitrogen for theactive layer.

An example of material preferable for the active layer is III-V compoundsemiconductors containing at least nitrogen (N), gallium (Ga) andarsenic (As). These III-V compound semiconductors have lattice constantsequal to or close to the lattice constant of GaAs and therefore can begrown on GaAs substrates with excellent crystalline quality. The activelayer made of the III-V compound semiconductor containing at leastnitrogen in the semiconductor optical device can be used to generatelight of a wavelength equal to or longer than 1 micrometer, therebyproviding 1 to 1.6 micrometer band light sources for opticalcommunications.

Typical examples of the III-V compound semiconductors containing atleast nitrogen, gallium and arsenic are GaNAs and GaInNAs. The III-Vcompound semiconductors containing at least nitrogen, gallium andarsenic can be lattice-matched to GaAs by adjusting their compositionsproperty. These III-V compound semiconductors are used as an activelayer for generating light of a long wavelength from 1 to 1.6micrometers.

The above III-V compound semiconductors can contain phosphorus and/orantimony in addition to the constituents of GaNAs or GaInNAs. Antimonycan work as surfactant and can suppress three-dimensional growth inGaNAs and GaInNAs crystal, thereby improving the crystal quality.Phosphorus can improve the crystal quality and reliability by reducingthe local crystal strain in GaNAs and GaInNAs. Besides, phosphoruscontributes to accelerating the introduction of nitrogen into the activelayer during crystal growth. Examples of material for the active layerare listed below: GaNAsP, GaInNAsP, GaNAsSb, GaInNAsSb, GaNAsSbP,GaInNAsSbP and so on. These III-V compound semiconductors have latticeconstants equal to or close to the lattice constant of GaAs andtherefore can be grown with excellent crystalline quality on GaAssubstrates or GaAs semiconductor.

If a GaAs substrate or a substrate of other material on which a GaAslayer is grown is used, the active layer can be made of III-V compoundsemiconductor, such as AlGaInP, GaInP, AlGaAs, GaAs, GaInAsP or GaInAs.These III-V compound semiconductors can have lattice constants close tothe lattice constant of GaAs by adjusting their compositions. TheseIII-V compound semiconductors are used as an active layer for generatinglight of a short wavelength from 0.6 to 1 micrometers.

Because the above materials for the active layer as above can be grownon GaAs surface or GaAs substrates, high band gap materials such asAlGaInP, GaInP, AlGaAs or GaInAsP lattice-matched to GaAs can be used asthe first and second conductive type semiconductor regions. The bandgapenergies of AlGaInP, AlGaAs and GaInAsP lattice-matched to GaAs aregreater than that of InP and these materials provide the followingbandgap energy ranges: 1.9 eV to 2.3 eV, 1.42 eV to 2.16 eV and 1.42 eVto 1.9 eV, respectively. GaInP lattice-matched to GaAs has the highbandgap energy of 1.9 eV.

Furthermore, the materials for the potential adjusting layer are asfollows: AlGaAs, GaAs, GaInAsP, GaInAs, AlGaInP, GaInP and so on. TheseIII-V compound semiconductors have lattice constants equal to or closeto the lattice constant of GaAs by adjusting their composition and thuscan be grown on GaAs substrates and GaAs semiconductor. As describedabove, since the potential adjusting layer may have a composition suchthat the lattice mismatch between the potential adjusting layer and thesubstrate or base layer is from −3% to +3%, the bandgap of AlGaInP,GaInP and GaInAsP can be more widely changed as compared with thebandgap of AlGaInP, GaInP and GaInAsP used for the first and secondconductive type semiconductor regions. Consequently, the bandgap energyof the potential adjusting layer can be widely changed in a range fromthe low bandgap energy of about 1 eV to the high bandgap energy of morethan 2.3 eV. Accordingly, the turn-on voltages V_(A) and V_(B) and theslope (series resistance) of the linear operation region in the currentvs. voltage characteristics can be set to be optimum by use of thepotential adjusting layer made of material appropriate to the intendeduse and by use of the first and second conductive type semiconductorregions having appropriate dopant concentration profiles as mentioned inthe ninth modified device, so that the semiconductor optical device canhave current vs. optical output power characteristics best fitted to theintended use.

If required, the semiconductor optical device has an optical confinementlayer(s). The material of the optical confinement layer is as follows:AlGaAs, GaAs and so on. Furthermore, the optical confinement layer canbe made of GaInAsP, AsGaInP, GaInP and so on which are lattice-matchedto GaAs.

In long wavelength band semiconductor optical devices having a GaAssubstrate or a GaAs-based layer, the above material the bandgap of whichcan be widely changed can be used for the first and second conductivetype semiconductor regions, the optical confinement layer and thepotential adjusting layer. Accordingly, the turn-on voltages V_(A) andV_(B) and the slope (series resistance) of the linear operation regionin the current vs. voltage characteristics can be set to be optimumeasily by use of the potential adjusting layer, the first and secondconductive type semiconductor regions, and the optical confinement layermade of the above materials appropriate to the intended use and by useof the first and second conductive type semiconductor regions in whichthe dopant concentration profiles are controlled appropriately asexplained in the ninth modified device, so that the semiconductoroptical device can have current vs. optical output power characteristicsbest fitted to the intended use. For example, in the semiconductoroptical device including the active layer made of III-V compoundsemiconductor containing at least nitrogen, gallium and arsenic, ifmaterials having a bandgap energy larger than that of InP are used forthe first conductive type semiconductor region, the second conductivetype semiconductor region and the potential adjusting layer providedbetween the second region of the first conductive type semiconductorregion and the second conductive type semiconductor region, then theturn-on voltage V_(B) can be increased compared with InP/GaInAsP basedlong wavelength semiconductor lasers without any potential adjustinglayers, whereby the linear operation region in the current vs. opticaloutput characteristics becomes wider and a higher output power can beobtained in the linear operation region. In addition, since the bandgapdifference between the active layer and the first conductive typesemiconductor region and second conductive type semiconductor regionbecomes greater compared with InP/GaInAsP based long wavelength buriedsemiconductor lasers, the confinement of carriers to the active layercan be enhanced, whereby the leakage current of the confined carriersfrom the active layer is reduced. Consequently, the lasing at highertemperatures can be achieved and thus temperature characteristics of thelong wavelength semiconductor optical device can be improved.

In the semiconductor optical device including the active layer made ofIII-V compound semiconductor not containing nitrogen, such as AlGaInP,GaInP, AlGaAs, GaAs, GaInAsP or GaInAs, if materials having the abovehigh bandgap energy are used for the first conductive type semiconductorregion, the second conductive type semiconductor region and thepotential adjusting layer provided between the second region of thefirst conductive type semiconductor region and the second conductivetype semiconductor region, then the turn-on voltage V_(B) can beincreased as compared to InP/GaInAsP based long wavelength semiconductorlasers without any potential adjusting layers. In this case, thematerials of the active layer and the optical confinement layer can beselected such that the turn-on difference (V_(B)−V_(A)) of the presentsemiconductor optical device becomes greater as compared to InP/GaInAsPbased long wavelength semiconductor lasers without any potentialadjusting layer. Consequently, the linear operation region becomes widercompared with InP/GaInAsP based long wavelength semiconductor laserswithout any potential adjusting layers and a higher output power can beobtained in the linear operation region of the current vs. opticaloutput power characteristics. Furthermore, since the bandgap differencebetween the active layer and the first and second conductive typesemiconductor regions becomes larger as compared to InP/GaInAsP basedlong wavelength semiconductor lasers without any potential adjustinglayers, the confinement of carriers to the active layer can be enhanced,whereby the confined carriers cannot be overflowed from the active layereasily. Consequently, the lasing at higher temperatures can be achievedand thus temperature characteristics of the semiconductor optical devicecan be improved.

Furthermore, since available GaAs substrates are large-sized such as 6inch in a diameter and are high quality and inexpensive, theproductivity improvement and cost reduction of the semiconductor opticaldevice are achieved and large-scaled integration of the semiconductoroptical device can be easily realized.

If the optical output power should be saturated in a low output region,the potential adjusting layer can be one of the following cases: thepotential adjusting layer provided between the active layer and thefirst and/or second conductive type semiconductor regions is made ofmaterial having a bandgap energy higher than the bandgap energies of thefirst and second conductive type semiconductor regions; the potentialadjusting layer provided between the second region of the firstconductive type semiconductor region and the second conductive typesemiconductor region is made of material having a bandgap energy lowerthan the bandgap energies of the first and second conductive typesemiconductor regions.

In the semiconductor optical device according to the present invention,InP substrates can be used or an InP layer grown on a substrate ofmaterial different from InP can be used. In this case, InP or AlGaInAslattice-matched to InP can be used for the material of the firstconductive type semiconductor region and the second conductive typesemiconductor region. If InP is used for the first conductive typesemiconductor region and the second conductive type semiconductorregion, then GaInAs, GaInAsP, and AlGaInAs can be used for material ofthe potential adjusting layer. If AlGaInAs is used for the firstconductive type semiconductor region and the second conductive typesemiconductor region, then GaInAs, GaInAsP, AlGaInAs and InP can be usedfor material of the potential adjusting layer. Since the latticeconstants of these materials have the same as or close to that of InP,these materials can be grown on the InP substrate or InP semiconductorregion grown on a substrate of material different from InP. For example,GaInAsP and AlGaInAs semiconductors with lattice-matched to InP havebandgap ranges of 0.74 eV to 1.35 eV and 0.74 eV to 1.5 eV,respectively, InGaAs with lattice-matched to InP has the bandgap energyof about 0.74 eV and InP has the bandgap energy of about 1.35 eV. If thestrained potential adjusting layer is allowed, the bandgap energy can bechanged more widely.

If an InP semiconductor layer is used for the material of the firstconductive type semiconductor region and the second conductive typesemiconductor region, GaInAs, GaInAsP and AlGaInAs each having the samelattice constant as or a lattice constant close to that of InP can beused for the active layer. If an AlGaInAs semiconductor layer can beused for the material of the first conductive type semiconductor regionand the second conductive type semiconductor region, GaInAs, GaInAsP,AlGaInAs and InP can be used for the active layer. The semiconductoroptical device includes a optical confinement layer(s) if required. Ifan InP semiconductor layer can be used for the material of the firstconductive type semiconductor region and the second conductive typesemiconductor region, GaInAs, GaInAsP and AlGaInAs lattice-matched toInP can be used for optical confinement layers. If an AlGaInAssemiconductor layer can be used for the material of the first conductivetype semiconductor region and the second conductive type semiconductorregion, GaInAs, GaInAsP, AlGaInAs and InP can be used for the opticalconfinement layer. Long wavelength band semiconductor lasers using theabove materials can generate light of 1 to 1.6 micrometer. In thesemiconductor lasers, the turn-on voltages V_(A) and V_(B) and the slope(series resistance) of the linear operation region in the current vs.voltage characteristics can be set to be optimum by use of the potentialadjusting layer made of material appropriate to the intended use and byuse of the first and second conductive type semiconductor regions inwhich the dopant concentration profiles are controlled appropriately asmentioned in the ninth modified device, so that the semiconductoroptical device can have the current vs. optical output powercharacteristics best fitted to the intended use.

In one example of the semiconductor optical device in which the firstconductive type semiconductor region and the second conductive typesemiconductor region are made of InP semiconductor and in which thepotential adjusting layer is provided between the second region of thefirst conductive type semiconductor region and the second conductivetype semiconductor region and includes the first region of the firstconductive type (the first region contacts with the first conductivetype semiconductor region) and the second region of the secondconductive type (the second region contacts with the second conductivetype semiconductor region), if these layers are made of AlGaInAs thebandgap of which is larger than that of InP, the built-in potential ofthe pn junction of this region becomes larger, thereby increasing theturn-on voltage V_(B) as compared to InP/GaInAsP based long wavelengthsemiconductor lasers without the potential adjusting layers. In anotherexample of the semiconductor optical device in which AlGaInAs is usedfor the first conductive type semiconductor region and the secondconductive type semiconductor region and in which the potentialadjusting layer is provided between the second region of the firstconductive type semiconductor region and the second conductive typesemiconductor region, if these layers are made of AlGaInAs having abandgap greater than that of InP, the turn-on voltage V_(B) is increasedcompared with the InP/GaInAsP based long wavelength semiconductor laserswithout the potential adjusting layer. In these cases, the turn-onvoltage difference V_(B)−V_(A) can be increased compared with theInP/GaInAsP based long wavelength semiconductor lasers without thepotential adjusting layers. Therefore, the linear operation region ofthe current vs. optical output power characteristics becomes wider and ahigher output power can be obtained in the linear operation regioncompared with the InP/GaInAsP based long wavelength semiconductor laserswithout the potential adjusting layers.

If the first conductive type semiconductor region and the secondconductive type semiconductor region are made of AlGaInAs having themaximum bandgap value of 1.5 eV, the bandgap energy differences betweenthe active layer and the first and the second conductive typesemiconductor regions becomes wider as compared to InP/GaInAsP basedlong wavelength buried semiconductor lasers. In this case, since theconfinement of carriers to the active layer can be enhanced, theconfined carriers cannot be overflowed from the active layer.Consequently, the lasing at higher temperatures can be achieved and thustemperature characteristics of the semiconductor optical device can beimproved.

If the optical output power should be saturated in a low output region,the potential adjusting layer can be one of the following cases: thepotential adjusting layer provided between the active layer and thefirst and/or second conductive type semiconductor regions is made ofmaterial having a bandgap energy larger than the bandgap energies of thefirst and second conductive type semiconductor regions; the potentialadjusting layer provided between the second region of the firstconductive type semiconductor region and the second conductive typesemiconductor region is made of material having a bandgap energy lowerthan the bandgap energies of the first and second conductive typesemiconductor regions.

In the semiconductor optical device according to the presentembodiments, the substrate can be one of the following substrates: GaNsubstrates; SiC substrates; Al₂O₃ substrates; Si substrates; AlNsubstrates; ZnO substrates; MgAl₂O₄ substrates. Alternatively, asemiconductor layer of one of these materials grown on a substrate ofmaterial different therefrom can be used as a base layer. In this case,the first and second conductive type semiconductor regions can be madeof AlGaN. The active layer and the potential adjusting layer can be madeof AlGaN, GaN or InGaN. If required, the semiconductor optical devicemay include an optical confinement layer(s). The optical confinementlayer is made of AlGaN GaN or InGaN. The lattice constant of AlGaN GaNor InGaN is the same as or close to that of GaN or AlN. Therefore, ifGaN substrates, Al substrates, GaN semiconductor layers and AlNsemiconductor layers are used, crystal defects due to the latticemismatch to the substrates are not generated during their growth, andthus the good crystal quality can be obtained. The bandgap of AlGaN canbe changed widely in a range from about 3.4 eV to 6.2 eV depending onthe composition. The bandgap of InGaN can be changed in a range fromabout 2 eV to 3.4 eV depending on the composition. In this semiconductorlaser, the turn-on voltages V_(A) and V_(B) and the slope (seriesresistance) of the linear operation region in the current vs. voltagecharacteristics can be set to be optimum by use of the potentialadjusting layer made of material appropriate to the intended use and byuse of the first and second conductive type semiconductor regions inwhich the doping concentration profiles are adjusted properly asexplained in the ninth modified device, so that the semiconductoroptical device can have current vs. optical output power characteristicsbest fitted to the intended use.

In addition for example, if the first and second conductive typesemiconductor regions and the potential adjusting layer provided betweenthe second region of the first conductive type semiconductor region andthe second conductive type semiconductor region are made of wide bandgapmaterials having nitride, the turn-on voltage V_(B) is increasedcompared with the InP/GaInAsP based long wavelength semiconductor laserswithout the potential adjusting layers. In this case, the materials ofthe active layer and the optical confinement layer can be selected suchthat the turn-on voltage difference (V_(B)−V_(A)) of the presentsemiconductor optical device becomes greater compared with theInP/GaInAsP based long wavelength semiconductor lasers without thepotential adjusting layers. Consequently, the linear operation region ofthe current vs. optical output power characteristics becomes wider ascompared to InP/GaInAsP based long wavelength semiconductor laserswithout the potential adjusting layers and a higher optical output powercan be obtained in the linear operation region. Furthermore, since thebandgap difference between the active layer and the first and secondconductive type semiconductor regions becomes larger compared with theInP/GaInAsP based long wavelength semiconductor lasers without thepotential adjusting layer, the confinement of carriers to the activelayer can be enhanced, whereby the confined carriers cannot come outfrom the active layer easily. Consequently, the lasing at highertemperatures can be achieved and thus temperature characteristics of thesemiconductor optical device can be improved. In addition, if the activelayer is made of AlGaN, GaN and InGaN, the semiconductor laser usingthis active layer can generate light in a wavelength region from blue toultraviolet.

Having described the first and second embodiments with reference to anumber of modifications, the present invention is not limited to theabove. In still another modified semiconductor optical device, the firstconductive type semiconductor region, the second conductive typesemiconductor region, and a potential adjusting layer, can be made ofmaterial not containing aluminum. In general, materials containingaluminum are oxidized easily. Therefore, if materials containingaluminum are used for the first conductive type semiconductor region,the second conductive type semiconductor region, and a potentialadjusting layer, interfaces among the first and second conductive typesemiconductor regions, a potential adjusting layer, an active layer andoptical confinement layers and interfaces among the first and secondconductive type semiconductor regions and a potential adjusting layerare oxidized during the device operation, whereby the number ofnonradiative recombination centers are increased. Consequently, theoptical characteristics and the reliability of the semiconductor opticaldevice are deteriorated. In addition, if the first conductive typesemiconductor region is made of material containing aluminum, thesurface of the first conductive type semiconductor region may be easilyoxidized and it is difficult to grow the second conductive typesemiconductor region thereon due to the surface oxidization. On theother hand, if the first and second conductive type semiconductorregions and a potential adjusting layer are made of materials notcontaining aluminum, the generation of nonradiative recombinationcenters at interface regions is avoided and the second conductive typesemiconductor region having excellent crystalline quality is grownthereon. Furthermore, if the second conductive type semiconductor regionis made of material not containing aluminum, the contact layer and theremaining of the second conductive type semiconductor region both havingexcellent crystalline quality are grown thereon in the second crystalgrowth step. For example, if a GaAs substrate is used, GaInP and GaInAsPcan be used as materials not containing aluminum for the first andsecond conductive type semiconductor regions, and GaInP, GaInAsP, GaAsand GaInAs can be used as materials not containing aluminum for thepotential adjusting layers.

The first conductive type semiconductor region has a part contacting thesecond conductive type semiconductor region (for example, the firstregion 54 a in FIG. 24) and the second conductive type semiconductorregion has a part contacting the first conductive type semiconductorregion (for example, the second region 58 a in FIG. 24). These parts andthe potential adjusting layer can be made of materials not containingaluminum. This structure provides the same advantages as those of thesemiconductor optical device in which the whole first conductive typesemiconductor region, the whole second conductive type semiconductorregion and the potential adjusting layer are made of materials notcontaining aluminum. Since the parts of the first and second conductivetype semiconductor regions that are not contacted with othersemiconductor regions can be made of materials containing aluminum, anymaterial can be used for these parts, which increases the flexibility indesigning semiconductor optical devices. Examples of material notcontaining aluminum are listed as follows: GaInP, GaAs, GaInAsP, GaInAsand so on.

In addition to the above structures, the active layer and opticalconfinement layers may be made of material not containing aluminum. Ifthese layers are made of material not containing aluminum, all thelayers in the semiconductor optical device do not contain aluminum.Then, this semiconductor optical device is free from aluminumoxidization related matters, thereby providing the semiconductor opticaldevice with higher performance and reliability. Examples of material forsemiconductor optical devices using GaAs substrates are as follows: thefirst and second conductive layers are made of GaInP or GaInAsP; thepotential adjusting layer is made of GaInAsP, GaAs, GaInAs or GaInP; theoptical confinement layers are made of GaAs or GaInAsP; the active layeris made of GaAs, GaInAs, GaInAsP or III-V compound semiconductorcontaining N, Ga and As.

The conductive type of the potential adjusting layer can be chosen from“p-type,” “n-type” and “undoped” in order to obtain a desired turn-onvoltage according to the intended use. The potential adjusting layer mayhave a multilayer structure including a plurality of films. The bandgapand conductivity of each film may be different from the others. In thiscase, turn-on voltages V_(A) and V_(B) and the series resistance afterthe turning-on of the current vs. voltage curve can be widely changedcompared with the potential adjusting layer of the single film, leadingto increasing flexibility in designing the current vs. optical outputcharacteristics.

If required, combinations of the potential adjusting layers in the aboveembodiments can be used to form another potential adjusting layer. Inthis case, turn-on voltages V_(A) and V_(B) and the series resistanceafter turning-on of the current vs. voltage output curve can be widelychanged compared with the potential adjusting layer of the single film,leading to increasing flexibility in designing the current vs. opticaloutput curve compared with the semiconductor optical device with thepotential adjusting layer of a single kind.

Furthermore, if the active layer has a quantum well structure, theactive layer may have a composition such that the lattice mismatchbetween the active layer and the substrate or the substrate or baselayer is from −3% to +3%. Since the thickness of the well layers can bevery thin and thinner than the critical thickness, the above range oflattice mismatch does not generate crystal defects such as misfitdislocation, and a good crystalline quality can be maintained. In thiscase, since the restriction on the lattice match condition between theactive layer and the substrate or the base layer is alleviated, theselayers can be made of a wider range of materials. Accordingly, thebandgap energy of the active layer can be changed more widely, leadingto more flexibility in designing the semiconductor optical devices.

Furthermore, although GaAs substrates, InP substrates, GaN substrates,Si substrate AlN substrate, ZnO substrate, sapphire (Al₂O₃) substrate,MgAl₂O₄ substrate and SiC substrates are listed in the above as examplesof the substrate, the semiconductor optical device according to thepresent invention can use other substrates. The first and secondconductive type semiconductor regions, the potential adjusting layer andactive layer can be grown on one of these substrates to form thesemiconductor optical device. In the case of sapphire (Al₂O₃) substrate,since it is an insulator different from other substrates, an electrode23 or 73 should be formed on the first conductive type semiconductorregion.

Having described and illustrated the principle of the invention in apreferred embodiment thereof, it is appreciated by those having skill inthe art that the invention can be modified in arrangement and detailwithout departing from such principles. For example, the semiconductoroptical device encompasses not only semiconductor lasers, but alsosemiconductor light-emitting diodes, semiconductor optical amplifiers,semiconductor electro-absorption modulators, semiconductor optical waveguide, semiconductor optical integrated devices and the like, as well asintegrated devices integrating these devices. Details of structures ofthese devices can be modified as necessary. We therefore claim allmodifications and variations coming within the spirit and scope of thefollowing claims.

1. A semiconductor optical device comprising: a first conductive typesemiconductor region having a first semiconductor portion and a secondsemiconductor portion, the first and second semiconductor portions beingprovided along a predetermined surface, the first semiconductor portionhaving a first region and a second region, the second semiconductorportion having a pair of sides, the second semiconductor portion beingprovided on the first region of the first semiconductor portion; anactive layer provided on the second semiconductor portion of the firstconductive type semiconductor region, the active layer having a pair ofsides; a second conductive type semiconductor region provided on thesides and top of the active layer, the sides of the second semiconductorportion, and the second region of the first semiconductor portion of thefirst conductive type semiconductor region, a bandgap energy of thefirst conductive type semiconductor region being greater than that ofthe active layer, a bandgap energy of the second conductive typesemiconductor region being greater than that of the active layer, thesecond region of the first semiconductor portion of the first conductivetype semiconductor region and the second conductive type semiconductorregion constituting a pn junction; and a potential adjustingsemiconductor layer provided between the second semiconductor portion ofthe first conductive type semiconductor region and the active layer, abandgap energy of the potential adjusting semiconductor layer beingdifferent from that of the first conductive type semiconductor region,and the bandgap energy of the potential adjusting semiconductor layerbeing different from that of the second conductive type semiconductorregion.
 2. A semiconductor optical device comprising: a first conductivetype semiconductor region having a first semiconductor portion and asecond semiconductor portion, the first and second semiconductorportions being provided along a predetermined surface, the firstsemiconductor portion having a first region and a second region, thesecond semiconductor portion having a pair of sides, the secondsemiconductor portion being provided on the first region of the firstsemiconductor portion; an active layer provided on the secondsemiconductor portion of the first conductive type semiconductor region,the active layer having a pair of sides; a second conductive typesemiconductor region having a top surface and a bottom surface, thesecond conductive type semiconductor region being provided on the sidesand top of the active layer, the sides of the second semiconductorportion, and the second region of the first semiconductor portion of thefirst conductive type semiconductor region, a bandgap energy of thefirst conductive type semiconductor region being greater than that ofthe active layer, a bandgap energy of the second conductive typesemiconductor region being greater than that of the active layer, thesecond region of the first semiconductor portion of the first conductivetype semiconductor region and the second conductive type semiconductorregion constituting a pn junction, the bottom surface of the secondconductive type semiconductor region being in contact with the sides ofthe second semiconductor portion and the second region of the firstsemiconductor portion of the first conductive type semiconductor region,the second conductive type semiconductor region covering the entire topof the active layer, and the second conductive type semiconductor regionbeing made of a single layer; a potential adjusting semiconductor layerprovided between the second conductive type semiconductor region and theactive layer, a bandgap energy of the potential adjusting semiconductorlayer being different from that of the first conductive typesemiconductor region, and the bandgap energy of the potential adjustingsemiconductor layer being different from that of the second conductivetype semiconductor region; a second conductive type semiconductorcontact layer in contact with the entire top surface of the secondconductive type semiconductor region; and an electrode in contact withthe second conductive type semiconductor contact layer.
 3. Asemiconductor optical device comprising: a first conductive typesemiconductor region having a first semiconductor portion and secondsemiconductor portion, the first and second semiconductor portions beingprovided along a predetermined surface, the first semiconductor portionhaving a first region and a second region, the second semiconductorportion having a pair of sides, the second semiconductor portion beingprovided on the first region of the first semiconductor portion; anactive layer provided on the second semiconductor portion of the firstconductive type semiconductor region, the active layer having a pair ofsides; a second conductive type semiconductor region provided on thesides and top of the active layer, the sides of the second semiconductorportion, and the second region of the first semiconductor portion of thefirst conductive type semiconductor region, a bandgap energy of thefirst conductive type semiconductor region being greater than that ofthe active layer, a bandgap energy of the second conductive typesemiconductor region being greater than that of the active layer, thesecond region of the first semiconductor portion of the first conductivetype semiconductor region and the second conductive type semiconductorregion constituting a pn junction; and a potential adjustingsemiconductor layer provided between the second conductive typesemiconductor region and the active layer, a bandgap energy of thepotential adjusting semiconductor layer being different from that of thefirst conductive type semiconductor region, and the bandgap energy ofthe potential adjusting semiconductor layer being different from that ofthe second conductive type semiconductor region, wherein the potentialadjusting semiconductor layer is provided between the secondsemiconductor portion of the first conductive type semiconductor regionand the active layer.
 4. A semiconductor optical device comprising: afirst conductive type semiconductor region having a first region and asecond region, the first and second regions being provided along apredetermined surface; an active layer provided on the first region ofthe first conductive type semiconductor region, the active layer havinga pair of sides; a second conductive type semiconductor region providedon the sides and top of the active layer, and the second region of thefirst conductive type semiconductor region; and a potential adjustingsemiconductor layer provided between the first region of the firstconductive type semiconductor region and the active layer, a bandgapenergy of the potential adjusting semiconductor layer being differentfrom that of the first conductive type semiconductor region, the bandgapenergy of the potential adjusting semiconductor layer being differentfrom that of the second conductive type semiconductor region, thebandgap energy of the first conductive type semiconductor region beinggreater than that of the active layer, the bandgap energy of the secondconductive type semiconductor region being greater than that of theactive layer, the second region of the first conductive typesemiconductor region and the second conductive type semiconductor regionconstituting a pn junction.
 5. A semiconductor optical devicecomprising: a first conductive type semiconductor region having a firstregion and a second region, the first and second regions being providedalong a predetermined surface; an active layer provided on the firstregion of the first conductive type semiconductor region, the activelayer having a pair of sides; a second conductive type semiconductorregion having a top surface and a bottom surface, the second conductivetype semiconductor region being provided on the sides and top of theactive layer, and the second region of the first conductive typesemiconductor region the bottom surface of the second conductive typesemiconductor region being in contact with the first conductive typesemiconductor region, the second conductive type semiconductor regioncovering the entire top of the active layer, and the second conductivetype semiconductor region being made of a single layer; a potentialadjusting semiconductor layer provided between the second conductivetype semiconductor region and the active layer, a bandgap energy of thepotential adjusting semiconductor layer being different from that of thefirst conductive type semiconductor region, and the bandgap energy ofthe potential adjusting semiconductor layer being different from that ofthe second conductive type semiconductor region, the bandgap energy ofthe first conductive type semiconductor region being greater than thatof the active layer, the bandgap energy of the second conductive typesemiconductor region being greater than that of the active layer, thesecond region of the first conductive type semiconductor region and thesecond conductive type semiconductor region constituting a pn junction;a second conductive type semiconductor contact layer in contact with theentire top surface of the second conductive type semiconductor region;and an electrode in contact with the second conductive typesemiconductor contact layer.
 6. A semiconductor optical devicecomprising: a first conductive type semiconductor region having a firstregion and a second region, the first and second regions being providedalong a predetermined surface; an active layer provided on the firstregion of the first conductive type semiconductor region, the activelayer having a pair of sides; a second conductive type semiconductorregion provided on the sides and top of the active layer, and the secondregion of the first conductive type semiconductor region; and apotential adjusting semiconductor layer provided between the secondconductive type semiconductor region and the active layer, a bandgapenergy of the potential adjusting semiconductor layer being differentfrom that of the first conductive type semiconductor region, and thebandgap energy of the potential adjusting semiconductor layer beingdifferent from that of the second conductive type semiconductor region,the bandgap energy of the first conductive type semiconductor regionbeing greater than that of the active layer, the bandgap energy of thesecond conductive type semiconductor region being greater than that ofthe active layer, the second region of the first conductive typesemiconductor region and the second conductive type semiconductor regionconstituting a pn junction, wherein the potential adjustingsemiconductor layer is provided between the first region of the firstconductive type semiconductor region and the active layer.
 7. Asemiconductor optical device comprising: a first conductive typesemiconductor region having a first region and a second region, thefirst and second regions being provided along a predetermined surface;an active layer provided on the first region of the first conductivetype semiconductor region, the active layer having a pair of sides; asecond conductive type semiconductor region provided on the sides andtop of the active layer, and the second region of the first conductivetype semiconductor region; and a potential adjusting semiconductor layerprovided between the second conductive type semiconductor region and theactive layer, a bandgap energy of the potential adjusting semiconductorlayer being different from that of the first conductive typesemiconductor region, and the bandgap energy of the potential adjustingsemiconductor layer being different from that of the second conductivetype semiconductor region, the bandgap energy of the first conductivetype semiconductor region being greater than that of the active layer,the bandgap energy of the second conductive type semiconductor regionbeing greater than that of the active layer, the second region of thefirst conductive type semiconductor region and the second conductivetype semiconductor region constituting a pn junction, wherein thebandgap energy of the potential adjusting semiconductor layer is smallerthan bandgap energies of the first and second conductive typesemiconductor regions, and the bandgap energy of the potential adjustingsemiconductor layer is larger than that of the active layer.
 8. Thesemiconductor optical device according to claim 5, wherein the bandgapenergy of the potential adjusting semiconductor layer is larger thanbandgap energies of the first and second regions.
 9. A semiconductoroptical device comprising: a first conductive type semiconductor regionhaving a first region and a second region, the first and second regionsbeing provided along a predetermined surface; an active layer providedon the first region of the first conductive type semiconductor region,the active layer having a pair of sides; a second conductive typesemiconductor region provided on the sides and top of the active layer,and the second region of the first conductive type semiconductor region;and a potential adjusting semiconductor layer provided between thesecond conductive type semiconductor region and the active layer, abandgap energy of the potential adjusting semiconductor layer beingdifferent from that of the first conductive type semiconductor region,and the bandgap energy of the potential adjusting semiconductor layerbeing different from that of the second conductive type semiconductorregion, the bandgap energy of the first conductive type semiconductorregion being greater than that of the active layer, the bandgap energyof the second conductive type semiconductor region being greater thanthat of the active layer, the second region of the first conductive typesemiconductor region and the second conductive type semiconductor regionconstituting a pn junction, wherein the first conductive typesemiconductor region includes a concentration changing region andanother region, the concentration changing region of the firstconductive type semiconductor region is provided between the otherregion of the first conductive type semiconductor region and the secondconductive type semiconductor region, the concentration changing regionof the first conductive type semiconductor region contacts with thesecond conductive type semiconductor region, and a dopant concentrationof the concentration changing region of the first conductive typesemiconductor region is different from the other region of the firstconductive type semiconductor region.
 10. A semiconductor optical devicecomprising: a first conductive type semiconductor region having a firstregion and a second region, the first and second regions being providedalong a predetermined surface; an active layer provided on the firstregion of the first conductive type semiconductor region, the activelayer having a pair of sides; a second conductive type semiconductorregion provided on the sides and top of the active layer, and the secondregion of the first conductive type semiconductor region; and apotential adjusting semiconductor layer provided between the secondconductive type semiconductor region and the active layer, a bandgapenergy of the potential adjusting semiconductor layer being differentfrom that of the first conductive type semiconductor region, and thebandgap energy of the potential adjusting semiconductor layer beingdifferent from that of the second conductive type semiconductor region,the bandgap energy of the first conductive type semiconductor regionbeing greater than that of the active layer, the bandgap energy of thesecond conductive type semiconductor region being greater than that ofthe active layer, the second region of the first conductive typesemiconductor region and the second conductive type semiconductor regionconstituting a pn junction, wherein the second conductive typesemiconductor region includes a concentration changing region andanother region, the concentration changing region of the secondconductive type semiconductor region is provided between the secondregion of the second conductive type semiconductor region and the firstconductive type semiconductor region, the concentration changing regionof the second conductive type semiconductor region contacts with thefirst conductive type semiconductor region, and a dopant concentrationof the concentration changing region of the second conductive typesemiconductor region is different from the other region of the secondconductive type semiconductor region.
 11. A semiconductor optical devicecomprising: a first conductive type semiconductor region having a firstsemiconductor portion and a second semiconductor portion, the first andsecond semiconductor portions being provided along a predeterminedsurface, the first semiconductor portion having a first region and asecond region, the second semiconductor portion having a pair of sides,the second semiconductor portion being located on the first region ofthe first semiconductor portion; an active layer provided on the secondsemiconductor portion of the first conductive type semiconductor region,the active layer having a pair of sides; a second conductive typesemiconductor region having a top surface and a bottom surface, thesecond conductive type semiconductor region being provided on the sidesand top of the active layer, the pair of sides of the secondsemiconductor portion, and the second region of the first semiconductorportion of the first conductive type semiconductor region, a bandgapenergy of the first conductive type semiconductor region being greaterthan that of the active layer, and a bandgap energy of the secondconductive type semiconductor region being greater than that of theactive layer, the bottom surface of the second conductive typesemiconductor region being in contact with the first conductive typesemiconductor region, the second conductive type semiconductorregioncovering the entire top of the active layer, and the second conductivetype semiconductor region being made of a single layer; a potentialadjusting semiconductor layer provided between the second region of thefirst semiconductor portion of the first conductive type semiconductorregion and the second conductive type semiconductor region, a bandgapenergy of the potential adjusting semiconductor layer being differentfrom that of the first conductive type semiconductor region, the bandgapenergy of the potential adjusting semiconductor layer being differentfrom that of the second conductive type semiconductor region, and thesecond region of the first semiconductor portion of the first conductivetype semiconductor region, the second conductive type semiconductorregion and the potential adjusting semiconductor layer being arranged toform a pn junction therein; a second conductive type semiconductorcontact layer in contact with the entire top surface of the secondconductive type semiconductor region; and an electrode in contact withthe second conductive type semiconductor contact layer.
 12. Asemiconductor optical device comprising: a first conductive typesemiconductor region having a first semiconductor portion and a secondsemiconductor portion, the first and second semiconductor portions beingprovided along a predetermined surface, the first semiconductor portionhaving a first region and second region, the second semiconductorportion having a pair of sides, the second semiconductor portion beinglocated on the first region of the first semiconductor portion; anactive layer provided on the second semiconductor portion of the firstconductive type semiconductor region, the active layer having a pair ofsides; a second conductive type semiconductor region provided on thesides and top of the active layer, the pair of sides of the secondsemiconductor portion, and the second region of the first semiconductorportion of the first conductive type semiconductor region, a bandgapenergy of the first conductive type semiconductor region being greaterthan that of the active layer, and a bandgap energy of the secondconductive type semiconductor region being greater than that of theactive layer; and a potential adjusting semiconductor layer providedbetween the second region of the first semiconductor portion of thefirst conductive type semiconductor region and the second conductivetype semiconductor region, a bandgap energy of the potential adjustingsemiconductor layer being different from that of the first conductivetype semiconductor region, the bandgap energy of the potential adjustingsemiconductor layer being different from that of the second conductivetype semiconductor region, and the second region of the firstsemiconductor portion of the first conductive type semiconductor region,the second conductive type semiconductor region and the potentialadjusting semiconductor layer being arranged to form a pn junctiontherein, wherein the potential adjusting semiconductor layer includes afirst region of a first conductive type and a second region of a secondconductive type, the first region and second region of the potentialadjusting semiconductor layer constitute the pn junction, the firstregion of the potential adjusting semiconductor layer and the secondregion of the first semiconductor portion of the first conductive typesemiconductor region constitute a junction, and the second region of thepotential adjusting semiconductor layer and the second conductive typesemiconductor region constitute a junction.
 13. A semiconductor opticaldevice comprising: a first conductive type semiconductor region having afirst region and a second region, the first and second regions beingprovided along a predetermined surface; an active layer provided on thefirst region of the first conductive type semiconductor region, theactive layer having a pair of sides; a second conductive typesemiconductor region having a top surface and a bottom surface, thesecond conductive type semiconductor region being provided on the sidesand top of the active layer, and the second region of the firstconductive type semiconductor region, bandgap energies of the first andsecond conductive type semiconductor regions being greater than abandgap energy of the active layer, the bottom surface of the secondconductive type semiconductor region being in contact with the firstconductive type semiconductor region, the second conductive typesemiconductor region covering the entire top of the active layer, andthe second conductive type semiconductor region being made of a singlelayer; a potential adjusting semiconductor layer provided between thesecond region of the first conductive type semiconductor region and thesecond conductive type semiconductor region, a bandgap energy of thepotential adjusting semiconductor layer being different from that of thefirst conductive type semiconductor region, the bandgap energy of thepotential adjusting semiconductor layer being different from that of thesecond conductive type semiconductor region, and the second region ofthe first conductive type semiconductor region, the second conductivetype semiconductor region and the potential adjusting semiconductorlayer being arranged to form a pn junction therein; a second conductivetype semiconductor contact layer in contact with the entire top surfaceof the second conductive type semiconductor region; and an electrode incontact with the second conductive type semiconductor contact layer. 14.A semiconductor optical device comprising: a first conductive typesemiconductor region having a first region and a second region, thefirst and second regions being provided along a predetermined surface;an active layer provided on the first region of the first conductivetype semiconductor region, the active layer having a pair of sides; asecond conductive type semiconductor region provided on the sides andtop of the active layer, and the second region of the first conductivetype semiconductor region, bandgap energies of the first and secondconductive type semiconductor regions being greater than a bandgapenergy of the active layer; and a potential adjusting semiconductorlayer provided between the second region of the first conductive typesemiconductor region and the second conductive type semiconductorregion, a bandgap energy of the potential adjusting semiconductor layerbeing different from that of the first conductive type semiconductorregion, the bandgap energy of the potential adjusting semiconductorlayer being different from that of the second conductive typesemiconductor region, and the second region of the first conductive typesemiconductor region, the second conductive type semiconductor regionand the potential adjusting semiconductor layer being arranged to form apn junction therein, wherein the potential adjusting semiconductor layerincludes a first region of a first conductive type and a second regionof a second conductive type, the first region and second region of thepotential adjusting semiconductor layer constitute the pn junction, thefirst region of the potential adjusting semiconductor layer and thesecond region of the first conductive type semiconductor regionconstitute a junction, and the second region of the potential adjustingsemiconductor layer and the second conductive type semiconductor regionconstitute a junction.
 15. A semiconductor optical device comprising: afirst conductive type semiconductor region having a first semiconductorportion and a second semiconductor portion, the first semiconductorportion having a first region and a second region, the first and secondregions being provided along a predetermined surface, the secondsemiconductor portion having a pair of sides, the second semiconductorportion being located on the first region of the first semiconductorportion; an active layer provided on the second semiconductor portion ofthe first conductive type semiconductor region, the active layer havinga pair of sides; a second conductive type semiconductor region providedon the sides and top of the active layer, the sides of the secondsemiconductor portion and the second region of the first semiconductorportion of the first conductive type semiconductor region, a bandgapenergy of the first conductive type semiconductor region being greaterthan that of the active layer and a bandgap energy of the secondconductive type semiconductor region being greater than that of theactive layer; and a potential adjusting semiconductor layer providedbetween the second region of the first semiconductor portion of thefirst conductive type semiconductor region and the second conductivetype semiconductor region and between the second semiconductor portionof the first conductive type semiconductor region and the active layer,a bandgap energy of the potential adjusting semiconductor layer beingdifferent from that of the second conductive type semiconductor region,and the second region of the first semiconductor portion of the firstconductive type semiconductor region, the second conductive typesemiconductor region and the potential adjusting semiconductor layerbeing arranged to form a pn junction therein.
 16. A semiconductoroptical device comprising: a first conductive type semiconductor regionhaving a first semiconductor portion and a second semiconductor portion,the first semiconductor portion having a first region and a secondregion, the first and second regions being provided along apredetermined surface, the second semiconductor portion being located onthe first region of the first semiconductor portion; an active layerprovided on the second semiconductor portion of the first conductivetype semiconductor region, the active layer having a pair of sides; asecond conductive type semiconductor region provided on the sides andtop of the active layer, the sides of the second semiconductor portionand the second region of the first semiconductor portion of the firstconductive type semiconductor region, a bandgap energy of the firstconductive type semiconductor region being greater than that of theactive layer and a bandgap energy of the second conductive typesemiconductor region being greater than that of the active layer; and apotential adjusting semiconductor layer provided between the secondregion of the first semiconductor portion of the first conductive typesemiconductor region and the second conductive type semiconductor regionand between the second conductive type semiconductor region and theactive layer, a bandgap energy of the potential adjusting semiconductorlayer being different from that of the first conductive typesemiconductor region, and the bandgap energy of the potential adjustingsemiconductor layer being different from that of the second conductivetype semiconductor region, and the second region of the firstsemiconductor portion of the first conductive type semiconductor region,the second conductive type semiconductor region and the potentialadjusting semiconductor layer being arranged to form a pn junctiontherein.
 17. The semiconductor optical device according to claim 16,wherein the potential adjusting semiconductor layer is provided betweenthe second semiconductor portion of the first conductive typesemiconductor region and the active layer.
 18. A semiconductor opticaldevice comprising: a first conductive type semiconductor region having afirst region and a second region, the first and second regions beingprovided along a predetermined surface; an active layer provided on thefirst region of the first conductive type semiconductor region, theactive layer having a pair of sides; a second conductive typesemiconductor region provided on the sides and top of the active layer,and the second region of the first conductive type semiconductor region,a bandgap energy of the first conductive type semiconductor region beinggreater than that of the active layer and a bandgap energy of the secondconductive type semiconductor region being greater than that of theactive layer; and a potential adjusting semiconductor layer providedbetween the second region of the first conductive type semiconductorregion and the second conductive type semiconductor region and betweenthe first region of the first conductive type semiconductor region andthe active layer, a bandgap energy of the potential adjustingsemiconductor layer being different from that of the first conductivetype semiconductor region, and the bandgap energy of the potentialadjusting semiconductor layer being different from that of the secondconductive type semiconductor region, and the second region of the firstconductive type semiconductor region, the second conductive typesemiconductor region and the potential adjusting semiconductor layerbeing arranged to form a pn junction therein.
 19. A semiconductoroptical device comprising: a first conductive type semiconductor regionhaving a first region and a second region, the first and second regionsbeing provided along a predetermined surface; an active layer providedon the first region of the first conductive type semiconductor region,the active layer having a pair of sides; a second conductive typesemiconductor region provided on the sides and top of the active layer,and the second region of the first conductive type semiconductor region,a bandgap energy of the first conductive type semiconductor region beinggreater than that of the active layer, and a bandgap energy of thesecond conductive type semiconductor region being greater than that ofthe active layer; and a potential adjusting semiconductor layer providedbetween the second region of the first conductive type semiconductorregion and the second conductive type semiconductor region and betweenthe second conductive type semiconductor region and the active layer, abandgap energy of the potential adjusting semiconductor layer beingdifferent from that of the first conductive type semiconductor region,the bandgap energy of the potential adjusting semiconductor layer beingdifferent from that of the second conductive type semiconductor region,and the second region of the first conductive type semiconductor region,the second conductive type semiconductor region and the potentialadjusting semiconductor layer being arranged to form a pn junctiontherein.
 20. The semiconductor optical device according to claim 19,wherein the potential adjusting semiconductor layer is provided betweenthe first region of the first conductive type semiconductor region andthe active layer.